tests/server: drop memdebug option

I added it just in case when removing enabled-by-default memdebug
from test servers. Apparently it broke after recent changes. It's
probably not a widely used feature and does not seem to be worth fixing.
It creates odd dependencies as the error message indicates:

```
[28/54] Building C object tests/server/CMakeFiles/servers.dir/__/__/lib/memdebug.c.obj
FAILED: tests/server/CMakeFiles/servers.dir/__/__/lib/memdebug.c.obj
[...]
lib/memdebug.c: In function 'curl_dbg_log':
lib/memdebug.c:465:12: error: implicit declaration of function 'mvsnprintf'; did you mean 'vsnprintf'? [-Wimplicit-function-declaration]
  465 |   nchars = mvsnprintf(buf, sizeof(buf), format, ap);
      |            ^~~~~~~~~~
      |            vsnprintf
lib/memdebug.c:465:12: warning: nested extern declaration of 'mvsnprintf' [-Wnested-externs]
```

This patch is dropping these build options:
- cmake: `ENABLE_SERVER_DEBUG`
- autotools: `--enable-server-debug` / `--disable-server-debug`

Follow-up to a16485a42e #16705

Closes #17629

docs/INSTALL-CMAKE.md

@@ -248,7 +248,6 @@ target_link_libraries(my_target PRIVATE CURL::libcurl)
 - `ENABLE_CURLDEBUG`:                       Enable TrackMemory debug feature. Default: =`ENABLE_DEBUG`
 - `ENABLE_CURL_MANUAL`:                     Build the man page for curl and enable its `-M`/`--manual` option. Default: `ON`
 - `ENABLE_DEBUG`:                           Enable curl debug features (for developing curl itself). Default: `OFF`
-- `ENABLE_SERVER_DEBUG`:                    Apply curl debug options to test servers. Default: `OFF`
 - `IMPORT_LIB_SUFFIX`:                      Import library suffix. Default: `_imp` for MSVC-like toolchains, otherwise empty.
 - `LIBCURL_OUTPUT_NAME`:                    Basename of the curl library. Default: `libcurl`
 - `PICKY_COMPILER`:                         Enable picky compiler options. Default: `ON`