Remove deprecated register

Summary:
In c++11, register has been deprecated, and
freetype is now built as gnu++11

From the documentation:
A register specifier is a hint to the implementation
that the variable so declared will be heavily used.
[ Note: The hint can be ignored and in most implementations
it will be ignored if the address of the variable is taken.
This use is deprecated (see D.2) ]

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D127