resourceloader: Remove 'mw.legacy' placeholder object

resourceloader: Remove 'mw.legacy' placeholder object

This was very briefly used during the development cycle of
MediaWiki 1.16 in 2010. The idea was to port some pre-RL legacy
modules from global variables to mw.legacy.* (such as sajax,
wikibits and other such 'mediawiki.legacy.*' modules),
with a back-compat deprecate-warning alias from their old global
names to the mw.legacy.* namespace.

This was abandoned prior to stable release and never used again,
and properly deprecated after years of no use in MediaWiki 1.22.

Remove this now, finally.

Change-Id: I9ef0984da7a07d4778fbeb71f3d4a7ebba97f29f