tests: Remove unused $wgMemc resets

If we really need this we can do it in MediaWikiTestCase, next
to the setting of wgMainCacheType. But from what I can see the
code being tested here already doesn't use the old $wgMemc.

Change-Id: I9e4b2109b2f3c18d8d5551bbadae5711c1d4c0a6