interwiki: Remove inappropiate use of wfMemcKey()

interwiki: Remove inappropiate use of wfMemcKey()

This is used for lookup in a CDB file or PHP static array.
In neither case is the key created by wfMemcKey() or any other
implementation of BagOStuff::makeKey().

This is already broken if:
* An interwiki prefix were to contain characters not supported by
  Memcached.
* An interwiki prefix and wikiid together are too long for Memcached.
* If the site has wgCachePrefix configured, which overrides the
  wfWikiID() namespace makeKey() normally uses.

dumpInterwiki.php does not use wfMemcKey() either (and should not).

This was simply here as leftover from many rewrites ago, its
only purpose is to create wikiid + prefix joined by colon.

Ref T148958.

Change-Id: I45682133ed593fbb0d66af5a67751f77f15a4a14