This commit removes ChatLog entirely. All of the application files are removed, and the migrations used are stubbed out. I stubbed the migrations as that allows for existing installs to make no changes, but new installs will not create the database.

Fixes T15126

Loaded up http://phorge.local/chatlog and confirmed the 404. Loaded up http://phorge.local/applications/view/PhabricatorChatLogApplication and confirmed the 404. Created a new database prefix and ran bin/storage upgrade against it, confirmed that the chatlog database was not created. Restored another prefix (an old one) and ran bin/storage upgrade and confirmed database was not deleted.