[NFC][emacs] remove out-of-date comment from tablegen-mode.el

Summary:
The syntax table was originally based on and attributed to
jasmin.el, but was rewritten in r45192, so the comment that
says the code comes from jasmin.el is no longer accurate. This
change removes the comment, shortening the code a bit.

Reviewers: MaskRay, lattner

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68042

llvm-svn: 373008