Revert "--enable-gio requires GVariant, which is >= 2.24"

It is actually only the shell/Library_losessioninstall.mk that requires
GVariant, and that one is Linux-only, all other GIO stuff is fine.
Which means now e.g. the ARM tinderbox fails. Revert, and either ignore
or wait for somebody to write a better check.

This reverts commit e5114027.

configure.ac

@@ -9361,7 +9361,7 @@ if test  "$test_gtk" = "yes"; then
             fi
             ENABLE_GIO="TRUE"
             AC_MSG_RESULT([yes])
-            PKG_CHECK_MODULES( GIO, gio-2.0 >= 2.24 )
+            PKG_CHECK_MODULES( GIO, gio-2.0 )
         else
             AC_MSG_RESULT([no])
         fi