Remove smwfLoadExtensionMessages

Was deprecated in 1.7 and now removed in 1.9 ald grep showed no
use in master

[1] https://gerrit.wikimedia.org/r/gitweb?p=mediawiki%2Fextensions%2FSemanticMediaWiki.git&a=search&h=HEAD&st=grep&s=smwfLoadExtensionMessages

Change-Id: If91bdece05cb1fa16dabe58d8b569abaaf5e6001