Remove unused XMALLOC_BLOCK_INPUT_CHECK debug facility

Remove unused XMALLOC_BLOCK_INPUT_CHECK debug facility

The compile-time option XMALLOC_BLOCK_INPUT_CHECK was added in
2012 (commit 4d7e6e51dd4acecff) to allow blocking input during
malloc-family calls, in case any issues arose from related
changes in Emacs 24.3.  However, it has not been referenced on
emacs-devel or the bug tracker in over a decade.

It is clear that we do not need it, as our signal handlers do not
allocate memory.  Removing it simplifies the allocation function
wrappers and eliminates dead debug code.

Ref: https://debbugs.gnu.org/12450

* src/alloc.c [XMALLOC_BLOCK_INPUT_CHECK]
(malloc_block_input, malloc_unblock_input): Delete functions.
(MALLOC_BLOCK_INPUT, MALLOC_UNBLOCK_INPUT): Delete macros.  Update
all callers.

(cherry picked from commit a7f5d183a8c13dfb7fb43777abdf96233df0ca0c)