MessageCache: Remove $wgMsgCacheExpiry configuration var

MessageCache: Remove $wgMsgCacheExpiry configuration var

This variable has never been set to anything other than the default value of
24 hours as introduced in 2003 (r2203, r2204; or 036ff960ce, edf6b38626).

The variable has never changed in core, it's not overridden at WMF,
and MessageCache is not constructed anywhere other than ServiceWiring.php
anywhere in repos on Wikimedia Gerrit, indexed by MediaWiki Codesearch,
or any GitHub-hosted repository (incl Wikia repos and WikiHow mirrors).

I've also checked all GitHub-hosted repos for boilerplates and/or public
settings files from devs or prod, and couldn't find any example of
this being overridden (after filtering out copies of the core files
themselves). Rather than having to support potentially hard-to-predict
interactions betweeen caching layers by checking its state, make it
a constant so we can code reason about it more easily.

Change-Id: Ie2e139001aae3ac54b509d94a3d917bb408eaca0