Disable AC_MSG_ERROR if gsettings-schema-convert is not found.

Reverted 729b0aa1febccbba97286f2aceb9044d907f9ffe
data/dconf/org.freedesktop.ibus.gschema.xml.in is included in tarball
so gsettings-schema-convert is not required.

BUG=http://code.google.com/p/ibus/issues/detail?id=1650

Review URL: https://codereview.appspot.com/58870044