cmake: drop HAVE_LIBWINMM and HAVE_LIBWS2_32 feature checks

- `HAVE_LIBWINMM` was detected but unused. The `winmm` system library is
  also not used by curl, but it is by its optional dependency `librtmp`.
  Change the logic to always add `winmm` when `USE_LIBRTMP` is set. This
  library has been available since the early days of Windows.

- `HAVE_LIBWS2_32` detected `ws2_32` lib on Windows. This lib is present
  since Windows 95 OSR2 (AFAIR). Winsock1 already wasn't supported and
  other existing logic already assumed this lib being present, so delete
  the check and replace the detection variable with `WIN32` and always
  add `ws2_32` on Windows.

Closes #11612

CMake/OtherTests.cmake

@@ -38,7 +38,7 @@ if(HAVE_WINDOWS_H)
   set(_source_epilogue
       "${_source_epilogue}\n#ifndef WIN32_LEAN_AND_MEAN\n#define WIN32_LEAN_AND_MEAN\n#endif")
   set(signature_call_conv "PASCAL")
-  if(HAVE_LIBWS2_32)
+  if(WIN32)
     set(CMAKE_REQUIRED_LIBRARIES ws2_32)
   endif()
 else()
@@ -133,4 +133,3 @@ if(NOT CMAKE_CROSSCOMPILING)
     }" HAVE_POLL_FINE)
   endif()
 endif()
-