[cmake] Remove MSVC C4800 override

Remove C4800 : ''type' : forcing value to bool 'true' or 'false' (performance warning)' from the list of forced disabled warnings.

I'm not seeing any regressions in VS2017/VS2019 llvm/clang builds from removing this.

Differential Revision: https://reviews.llvm.org/D61730

llvm-svn: 360417