SurfaceFlinger: Fix compile error by removing unneeded HWC_BLIT

Change-Id: I6f4ac4a76559602cae95eb8ba2973b131d5855c4

I'm compiling Jellybean 4.2.2 for the HTC Thunderbolt and this variable was causing compiler errors (undefined). I'm not certain if this is device specific or if it is because I am missing a header include that defines the HWC_BLIT variable. If you need additional information I will be happy to provide it. The build completes and performs just fine when installed.