docs: remove the references to Z-SAMPA.

Z-SAMPA is a more complex specification than Kirshenbaum, X-SAMPA,
and CXS. Additionally, it specifies phonemes such as palatal and
velar trills that are not defined by IPA and in the context of the
velar trill is marked as being impossible.

As such, Z-SAMPA will not be supported in eSpeak NG for now. That
may change in the future once support for the other transcription
schemes has been implemented.

docs/phonemes.md

 | [`fonxsamp`](phonemes/xsampa.md)        | X-SAMPA      | Extended Speech Assessment Methods Phonetic Alphabet | ASCII    |
 | [`x-foncxs`](phonemes/cxs.md)           | CXS          | Conlang X-SAMPA                                      | ASCII    |
 | [`x-fonkirsh`](phonemes/kirshenbaum.md) |              | Kirshenbaum (ASCII-IPA)                              | ASCII    |
-| `x-fonzsamp`                            | Z-SAMPA      | Zompist Bulletin Board (ZBB) SAMPA                   | ASCII    |
 
-1. `foncxs`, `fonkirsh`, and `fonzsamp` are private use extensions defined in
+1. `foncxs` and `fonkirsh` are private use extensions defined in
    the [bcp47-extensions](https://raw.githubusercontent.com/espeak-ng/bcp47-data/master/bcp47-extensions)
    file, so have the `x-` private use specifier before their subtag names.
 

docs/phonemes/cxs.md

 | [`fonxsamp`](xsampa.md)        | X-SAMPA      | Extended Speech Assessment Methods Phonetic Alphabet | ASCII    |
 | `x-foncxs`                     | CXS          | Conlang X-SAMPA                                      | ASCII    |
 | [`x-fonkirsh`](kirshenbaum.md) |              | Kirshenbaum (ASCII-IPA)                              | ASCII    |
-| `x-fonzsamp`                   | Z-SAMPA      | Zompist Bulletin Board (ZBB) SAMPA                   | ASCII    |
 
-1. `foncxs`, `fonkirsh`, and `fonzsamp` are private use extensions defined in
+1. `foncxs` and `fonkirsh` are private use extensions defined in
    the [bcp47-extensions](https://raw.githubusercontent.com/espeak-ng/bcp47-data/master/bcp47-extensions)
    file, so have the `x-` private use specifier before their subtag names.
 

docs/phonemes/kirshenbaum.md

 | [`fonxsamp`](xsampa.md)    | X-SAMPA      | Extended Speech Assessment Methods Phonetic Alphabet | ASCII    |
 | [`x-foncxs`](cxs.md)       | CXS          | Conlang X-SAMPA                                      | ASCII    |
 | `x-fonkirsh`               |              | Kirshenbaum (ASCII-IPA)                              | ASCII    |
-| `x-fonzsamp`               | Z-SAMPA      | Zompist Bulletin Board (ZBB) SAMPA                   | ASCII    |
 
-1. `foncxs`, `fonkirsh`, and `fonzsamp` are private use extensions defined in
+1. `foncxs` and `fonkirsh` are private use extensions defined in
    the [bcp47-extensions](https://raw.githubusercontent.com/espeak-ng/bcp47-data/master/bcp47-extensions)
    file, so have the `x-` private use specifier before their subtag names.
 

docs/phonemes/xsampa.md

 | `fonxsamp`                     | X-SAMPA      | Extended Speech Assessment Methods Phonetic Alphabet | ASCII    |
 | [`x-foncxs`](cxs.md)           | CXS          | Conlang X-SAMPA                                      | ASCII    |
 | [`x-fonkirsh`](kirshenbaum.md) |              | Kirshenbaum (ASCII-IPA)                              | ASCII    |
-| `x-fonzsamp`                   | Z-SAMPA      | Zompist Bulletin Board (ZBB) SAMPA                   | ASCII    |
 
-1. `foncxs`, `fonkirsh`, and `fonzsamp` are private use extensions defined in
+1. `foncxs` and `fonkirsh` are private use extensions defined in
    the [bcp47-extensions](https://raw.githubusercontent.com/espeak-ng/bcp47-data/master/bcp47-extensions)
    file, so have the `x-` private use specifier before their subtag names.