Revert "Add TeX annotation to MathML again"

There is no proper escaping for these annotations yet (which breaks math
rendering in production & introduces a security hole), and it is not
completely clear if the MML produced in this mode is actually expected to be
valid display MML at all.

Lets fix & check those things first before re-enabling annotations.

This reverts commit a350d2a271c9031d7c9a5fc2a97c64fb422fd7ff.

Change-Id: I88c8d8c671e3916f6b043b5cd33810fe3babb425