build: Don't compile with _FILE_OFFSET_BITS=64 on Windows (MinGW)

* cmake/modules/G10CompilerSettings.cmake: Remove adding definition of
-D_FILE_OFFSET_BITS=64 for MinGW.

This reverts commit 629daa8492de4c0a42e4a3be31eafad072a4524b.

Defining _FILE_OFFSET_BITS=64 caused an incompatibility with gpgme for
32-bit builds which resulted in crashes. gpgme is explicitly built
without _FILE_OFFSET_BITS=64 on Windows.