Sync exif dependency on mbstring extension as optional (#3793)

The required dependency on mbstring extension in exif was removed via
php/php-src@755c2cd which made the
mbstring extension optional dependency.

Update for the PHP build system:
php/php-src#16062

Author: petk

reference/exif/setup.xml

@@ -13,11 +13,6 @@
    extension must be enabled by compiling PHP with <literal>--enable-mbstring</literal>.
    PHP does not require any additional library for the exif module.
   </para>
-  <para>
-   Windows only: The <link linkend="ref.mbstring">mbstring</link> extension must always
-   be enabled. Note that <link linkend="ref.mbstring">mbstring</link> extension must be
-   loaded prior to EXIF in <filename>php.ini</filename>.
-  </para>
  </section>
  <!-- }}} -->