Cleanup: remove _MAYBENULL helpers

[EgorBo]

 CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE
-CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE_MAYBENULL
 CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE
-CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPEHANDLE_MAYBENULL

So instead of

CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE_MAYBENULL(handle)

codegenerator should use

handle == 0 ? null : CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE(handle)

This simplifies things and enables it to be constant-folded later when jit can learn something about handle's nullness

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