tests/server: drop memdebug option

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 a16485a42ea5dabe6c327179a1678ad04d1c6b2f #16705

Closes #17629