Remove Autoconf check for C99 header stdint.h (#13444)

This removes the redundant check that defines the HAVE_STDINT_H symbol,
which is used in the ext/date/lib and ext/fileinfo/libmagic but on both
places patches and other header files take care of this so it is always
included unconditionally in php-src.

Autoconf by default still automatically checks for stdint.h internally
in current versions, so for now the HAVE_STDINT_H symbol is still
defined in php_config.h and will be made redundant along the way.

Author: petk

configure.ac

@@ -383,7 +383,6 @@ dnl ----------------------------------------------------------------------------
 
 dnl QNX requires unix.h to allow functions in libunix to work properly.
 AC_CHECK_HEADERS([ \
-stdint.h \
 dirent.h \
 sys/param.h \
 sys/types.h \