warnless: drop parts of the read/write preprocessor hack (Windows)

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 6239146e93

Closes #17673

lib/curlx/warnless.c

@@ -35,11 +35,6 @@
 
 #endif /* __INTEL_COMPILER && __unix__ */
 
-#ifdef _WIN32
-#undef read
-#undef write
-#endif
-
 #include <limits.h>
 
 #define CURL_MASK_UCHAR   ((unsigned char)~0)
@@ -295,7 +290,3 @@ size_t curlx_sitouz(int sinum)
 #  pragma warning(pop)
 #endif
 }
-
-/* Ensure that warnless.h redefinitions continue to have an effect
-   in "unity" builds. */
-#undef HEADER_CURL_WARNLESS_H_REDEFS

lib/curlx/warnless.h

@@ -57,11 +57,6 @@ unsigned short curlx_uitous(unsigned int uinum);
 
 size_t curlx_sitouz(int sinum);
 
-#endif /* HEADER_CURL_WARNLESS_H */
-
-#ifndef HEADER_CURL_WARNLESS_H_REDEFS
-#define HEADER_CURL_WARNLESS_H_REDEFS
-
 #ifdef _WIN32
 #undef  read
 #define read(fd, buf, count)  (ssize_t)_read(fd, buf, curlx_uztoui(count))
@@ -69,4 +64,4 @@ size_t curlx_sitouz(int sinum);
 #define write(fd, buf, count) (ssize_t)_write(fd, buf, curlx_uztoui(count))
 #endif
 
-#endif /* HEADER_CURL_WARNLESS_H_REDEFS */
+#endif /* HEADER_CURL_WARNLESS_H */