gpu: vivante: Removed definitions of unused macros

There are more unused macros, but those are part of a set where other
macros in the set are in use, so I decided not to remove those.
gpu: vivante: Removed declarations of unimplemented functions

gckKERNEL_GetVideoMemoryPoolPid and gckKERNEL_CreateVideoMemoryPoolPid
are also unimplemented, but those are called if gcdUSE_VIDMEM_PER_PID
is enabled, so I left their declaration in place for now.