Do not customize __format__ for Windows/MinGW to gnu_printf

Bug: http://b/69933090
Bug: http://b/69933068

MinGW uses gnu_printf to force C99 printf analysis with GCC, but clang
does not support gnu_printf.  So just use the default that's used for
other platforms.  This also mirrors upstream commit
https://sourceforge.net/p/mingw-w64/mingw-w64/ci/015e637b4b24d9915162ab877ed539ad0e657951/.

Test: m native-host-cross with Clang.
Change-Id: I3deb266d70e25296c4ae1d58637afbabc3d949c1