Remove aarch64_cannot_change_mode_class as the underlying issue (PR67609) has been resolved.

Remove aarch64_cannot_change_mode_class as the underlying issue
(PR67609) has been resolved.  This avoids a few unnecessary lane
widening operations like:

faddp   d18, v18.2d
mov     d18, v18.d[0]

    gcc/
	PR67609
	* config/aarch64/aarch64.h (CANNOT_CHANGE_MODE_CLASS): Remove.
	* config/aarch64/aarch64.c
	(aarch64_cannot_change_mode_class): Remove function.
	* config/aarch64/aarch64-protos.h
	(aarch64_cannot_change_mode_class): Remove.

From-SVN: r236817