Options.td: remove a_Group and L_Group.

They don't seems to serve any purpose.

Differential Revision: http://llvm-reviews.chandlerc.com/D1227

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@187236 91177308-0d34-0410-b5e6-96231b3b80d8