The `<assert.h>` header file is part of the standard C89 headers [1] and
on older systems there needed to be also a manual check if header is
present.
Since PHP requires at least C89 manual check and the `HAVE_ASSERT_H`
symbol defined by Autoconf in configure.ac can be both removed [2].
This patch also removes unused <assert.h> includes where c files don't
use the `assert()` macro.
Refs:
[1] https://port70.net/~nsz/c/c89/c89-draft.html#4.2
[2] https://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/headers.m4
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