Patch to remove:
https://gerrit.wikimedia.org/r/c/mediawiki/core/+/447647
Bug:
https://phabricator.wikimedia.org/T200247
Existing uses:
https://codesearch.wmflabs.org/search/?q=MagicWord%3A%3AclearCache&i=no…
I'm in the middle of creating a MagicWordFactory service to replace
the MagicWord static methods. This obsoletes the clearCache() method.
Instead, just reset the service with resetServiceForTesting(). Since
clearCache() is used in only three places in public code, and is only
useful to begin with for tests, and the fix is trivial, I propose
removing it without deprecation rather than bothering to maintain code
for compatibility.