Remove support for the QA_OVERRIDE_GCC3_OPTIONS environment variable.

In r199283 I switched the name of this variable to CCC_OVERRIDE_OPTIONS, but
I kept some code to continue recognizing the old name temporarily. As far as
I know, the only use of this was for some internal testing at Apple, and we've
now switched to use the new name. If anyone else is still using this and needs
more time to switch names, I guess we'll find out! <rdar://problem/15821425>