Revert "glib-init: Add more static assertions for gintptr, guintptr"

On closer inspection, these are redundant with stricter assertions that I
added in commit bd1e2a98 "glib-init: Statically assert more facts about
standard types", which assert that sizeof (gintptr) == sizeof (void *).

As far as I can tell, a sufficiently pedantic interpretation of Standard
C doesn't actually require (u)intptr_t to be the same size as a pointer:
it only requires that pointers can be losslessly stored in a (u)intptr_t,
which a sufficiently pathological ABI could implement by having (for
example) 32-bit pointers and a 64-bit uintptr_t just to troll us. However,
I'm fairly confident that no practically useful ABI would do this,
and certainly nobody has complained about this assertion since 2020.

Similarly, Standard C might permit an ABI where 64-bit pointers have the
first 32 bits always-zero and therefore storing the remaining bits in a
32-bit uintptr_t is lossless, but again, that would be pathological.

This reverts commit da3fc595

.

Signed-off-by: Simon McVittie <smcv@collabora.com>