Revert "(bug 1495) Enable on-wiki message language fallbacks"

This reverts commit d434bfcf3bbab05660ed8f798a4622487dd8ba56.

See Iac7ac4357dd80e8cdb238a6a207393f0712b3fe5.

Conflicts:
	includes/cache/MessageCache.php
	tests/phpunit/includes/cache/MessageCacheTest.php

Change-Id: I5ea8af299a14e052b265ebf9f21914ab0a4eb922
Merge "Revert "(bug 1495) Enable on-wiki message language fallbacks"" into REL1_21