Revert "Highlight LaTeX blocks delimited by $ on line alone"

Remove multiline tex highlighting with $ blocks. Although it looks nice,
it isn't recognized by pandoc rendering so literal $ are typeset.

This reverts commit 42fc3987175340ecf41c077bb33517bb12141db4.
Toggle pandoc/tex syntax highlighting w/ local mapping

Toggle between pandoc & tex syntax highlight by <localleader>s

Mixed syntax highlighting is painful, and imperfect at best, so a
workaround is to simply switch syntax highlighting modes manually when
editing large blocks of tex or pandoc