Endgame code refactor

This is mostly just some code cleanup and moving about:

• Y_EndGame was moved from from y_inter.c/h to g_game.c/h, and renamed to G_EndGame. Frankly the old place that function lived didn't make sense, since it has nothing to do with tally screens directly.
• Most of Y_FollowIntermission's code was removed, because G_AfterIntermission does it all already.
• And lastly, G_ExitLevel uses HU_ClearCEcho() instead of HU_CEcho("") to clear CEchos from the screen.

The only visible changes in-game from the above should be just that log.txt stops adding empty new lines when you complete a level, and CEchos will always be cleared after tally screens when a level goes to credits/evaluation next.

(Backport from 2.2)