Remove HAVE_ASSERT_H

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

ext/bcmath/libbcmath/src/num2str.c

@@ -31,7 +31,6 @@
 
 #include <config.h>
 #include <stdio.h>
-#include <assert.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <stdarg.h>