Remove 64 bit pipeline

Remove 64 bit pipeline

The 64 bit pipeline is not used anymore, so it can now be removed.

Don't generate pixman-combine64.[ch] anymore. Don't generate the
pixman-srgb.c anymore. Delete all the 64 bit fetchers in
pixman-access.c, all the 64 bit iterator functions in
pixman-bits-image.c and all the functions that expand from 8 to 16
bits.