This removes one member of GHOST_ImeWin32 that is not used and cannot
be used in the future. It is holding the result of ImmIsIME, which is
whether an input language supports IME. It does not indicate that one
is in use, turned on, composing, in English mode, etc.
see D12131 for more information.
Differential Revision: https://developer.blender.org/D12131
Reviewed by Ray Molenkamp