gimple: Remove custom remove_pointer

gimple: Remove custom remove_pointer

Since r11-2700-g22dc89f8073cd0, type_traits has been included via system.h so
we don't need a custom version for gimple.h.

Note a small C++14 cleanup is to use remove_pointer_t directly here instead
of remove_pointer<t>::type.

bootstrapped and tested on x86_64-linux-gnu

gcc/ChangeLog:

	* gimple.h (remove_pointer): Remove.
	(GIMPLE_CHECK2): Use std::remove_pointer instead of custom one.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>