Remove PMSIHANDLE struct

[AArnott]

The PMSIHANDLE struct is defined in the metadata as:

public struct PMSIHANDLE
{
	public MSIHANDLE m_h;
}

As we see, PMSIHANDLE is defined as just having MSIHANDLE as a field. This is not a pointer, nor any different from using MSIHANDLE directly except to make accessing the inner one more tedious. Can we remove the P version?