Don't link "self::" and "parent::" as symbols in PhutilXHPASTSyntaxHighlighter

Summary:
We now cross-link from Differential to Diffusion, but incorrectly link "self::"
and "parent::", which will never go anywhere useful.

(One might imagine a future world where we are smart enough with static analysis
that we take you to the right place for these symbols, but we aren't there yet,
and unlinking them is better than linking them badly.)

Test Plan: Ran unit test. Uploaded a diff which uses "self::" and "parent::" and
they were not linked.

Reviewers: btrahan, jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: aran, jungejason, epriestley

Differential Revision: 1121