Remove orphaned gfx::text::shaper code

The shaper code referenced in this issue is no longer being used.  It
was removed in commit a535f221469940d9549550953b845010a353d3bc, but
later reintroduced due to a merge conflict in commit
d64d987e1da66691ccd0e7e04fd54babc2c5bab4.

Fixes #163.