Member access is `->` followed by lvar, not word

Summary: Minor parser tweak; you're not allowed `-` in variable names.
See comment in new lexer rule.

Also removes a couple unused definitions while I'm at it.

Fixes https://github.com/facebook/hhvm/issues/3712

Reviewed By: @int3

Differential Revision: D1683429

Signature: t1:1683429:1416113309:0098f9c4715bd9f6e3d5cc5d13d744a24d4c9c22