Remove PlantUML and Math copy-pasted lexer extensions
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8588 and (???), the Plaintext rouge lexer was copy-pasted into our repository. There was no reason for this other than misunderstanding of how our Banzai filters and triple-backtick syntax work. They recently broke in the upgrade to rouge 2.1. They should be removed.
Edited by jneen