content: Deprecate TitleIsCssOrJsPage and TitleIsWikitextPage hooks

They're currently documented as a 'compatibility hook'
in docs/contenthandler.txt. No longer used in any Wikimedia-hosted
git repository.

Superseded by the ContentHandlerDefaultModelFor hook.

Change-Id: I212230da7d6080cf500f930d4aa5a9024959d5f9
Merge "content: Deprecate TitleIsCssOrJsPage and TitleIsWikitextPage hooks"