php: remove obsolete mbstring module

This module was used in the old XMLRPC client code, but is no longer
needed.  Removal was escalated by FreeBSD when the oniguruma4 dependency
of the php56-mbstring module was marked BROKEN.

(cherry picked from commit 3da643e0)

Makefile

@@ -85,7 +85,6 @@ CORE_DEPENDS?=		apinger \
 			php56-hash \
 			php56-json \
 			php56-ldap \
-			php56-mbstring \
 			php56-mcrypt \
 			php56-mysql \
 			php56-openssl \

src/etc/rc.php_ini_setup

@@ -50,8 +50,6 @@ PHPMODULES="$PHPMODULES session"
 PHPMODULES="$PHPMODULES suhosin"
 # Firewall rules edit
 PHPMODULES="$PHPMODULES ctype"
-# firewall_rules_edit.php
-PHPMODULES="$PHPMODULES mbstring"
 # Page compression
 PHPMODULES="$PHPMODULES zlib"
 # SQLlite & Database