The `#undef` hack is no longer necessary after changing the redifitions
to not map back to the original symbols.
This makes it unnecessary to repeat the redefinitions after compiling
`warnless.c` itself (in unity mode).
Which in turns makes it unnecessary to include `warnless.h` again, to
trigger such redefinition.
This also means that `read`/`write` are now redefined on Windows from
the first inclusion of `warnless.h`.
Also:
- tests/server: drop a repeat `warnless.h` include, that is unnecessary
after this patch.
- tests/unit: drop repeat `warnless.h` include.
- tests/libtest: drop repeat `warnless.h` includes.
- tests/libtest: formatting.
Follow-up to 2f312a14da#17619
Follow-up to 84338c4de2#12331
Follow-up to 6239146e93Closes#17673