Minor cleanup.
Removed dead code.

Stuff that won't be present on ES targets anyway because it's already
disabled somewhere else.
There is glDiscardSubFramebuffer() in EXT_discard_framebuffer.

The relevant functions are now a no-op on ES2.
Decide between ANGLE/NV implementation for Framebuffer::blit() on ES.

Currently both disabled until extension loader is in place.