terminalapp: delete plugin deprecated since 2015 (#8230)

plugins/terminalapp/terminalapp.plugin.zsh

@@ -1,6 +0,0 @@
-# This file is intentionally empty.
-#
-# The terminalapp plugin is deprecated and may be removed in a future release.
-# Its functionality has been folded in to the core lib/termsupport.zsh, which
-# is loaded for all users. You can remove terminalapp from your $plugins list
-# once all your systems are updated to the current version of Oh My Zsh.