Revert "test: coverage for wfWikiID()"

Revert "test: coverage for wfWikiID()"

Cause unit tests to fail for some reason:

 1) WfWikiId::testReturnsProperDbName
 Failed asserting that two strings are equal.
 --- Expected
 +++ Actual
 @@ @@
 -'known_db_name'
 +'known_db_name-unittest_'

 tests/phpunit/includes/GlobalFunctions/wfWikiIDTest.php:10
 tests/phpunit/MediaWikiTestCase.php:133
 tests/phpunit/MediaWikiPHPUnitCommand.php:42

Thus making the regression job to complain whenever a patch is merged:
https://integration.wikimedia.org/ci/job/mediawiki-core-regression-master/

This reverts commit 4f55b804f20960e039671a3dbf3f3eb4a5a4b81e.

Change-Id: Ib08e40742b74be88b72af79ec4ab6d427e3b6d14