Merge view/init.mw.php into view/WikibaseView.php

Wikibase houses four virtual MediaWiki extensions:

• Wikibase Client (client/WikibaseClient.php)
• Wikibase Repo (repo/Wikibase.php)
• Wikibase Lib (lib/WikibaseLib.php)
• Wikibase View (view/WikibaseView.php)

However, in the case of Wikibase View, there was also a separate file
entry point "init.mw.php", which appears to have no added purpose.
This commit merges it into the main entry point, matching the way
the other three extensions are initialised.

Bug: T189538
Change-Id: I2cd1db24b3e2d174b4262ddff69445e89bc064e5