Remove unsuitable compiler flag

Control of multi-core compilation should be set at the build tool (e.g. MSBuild, ninja, make)
and not as a compiler flag.

Also cmake already append it automatically:
https://github.com/Kitware/CMake/blob/e75afeae5cc1b41e1fb97e1924e6dbcd1e39d91a/CompileFlags.cmake#L118-L134

CMakeLists.txt

@@ -612,14 +612,14 @@ if (WIN32)
  )
  target_compile_options(torrent-rasterbar
  PRIVATE
+ # allow larger .obj files (with more sections)
+ /bigobj
  # https://learn.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170
  /permissive-
  # https://docs.microsoft.com/en-us/cpp/build/reference/utf-8-set-source-and-executable-character-sets-to-utf-8?view=msvc-170
  /utf-8
  # https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
  /Zc:__cplusplus
- /MP # for multi-core compilation
- /bigobj # increase the number of sections for obj files
  )
  set_target_properties(torrent-rasterbar PROPERTIES LINK_FLAGS_RELEASE "/OPT:ICF=5 /OPT:REF")
  endif()