Autotools: Remove too basic optimization flag cleanup

The optimization flags are removed in configure.ac when using the
'--enable-debug' configure option (which also adds the '-O0'). When
using '--enable-debug-assertions' option, the optimization flags ideally
shouldn't be removed and this case never actually happen because the
CFLAGS at this point in ZEND_INIT contain all sorts of other flags also,
so it's redundant as it never gets executed.

Closes phpGH-15305

Author: petk

Zend/Zend.m4

@@ -177,9 +177,6 @@ AH_TEMPLATE([ZEND_DEBUG],
 AS_VAR_IF([ZEND_DEBUG], [yes], [
   AC_DEFINE([ZEND_DEBUG], [1])
   echo " $CFLAGS" | grep ' -g' >/dev/null || CFLAGS="$CFLAGS -g"
-  if test "$CFLAGS" = "-g -O2"; then
-    CFLAGS=-g
-  fi
 ], [AC_DEFINE([ZEND_DEBUG], [0])])
 
 AS_VAR_IF([GCC], [yes],