SCUBA2: remove long-deprecated primitive

The primitive _DISPLAY_FASTFLAT_ superceded this one way back in 2011,
but it's only just now that I remembered that fact. I think it's time
to delete it...
SCUBA2: remove mention of PROCESS_FASTFLAT uhdr

The PROCESS_FASTFLAT uhdr is no longer used, having been replaced by
an appropriate tag.
SCUBA2: remove PROCESS_FASTFLAT uhdr

The PROCESS_FASTFLAT uhdr has been phased out and is no longer used,
despite it being set in these primitives. Therefore it's time to
remove all reference to it.