PICARD: use custom primitive for deleting files

The _SCUBA2_TIDY_ primitive has a bit more flexibility when it comes
to deleting intermediate files from SCUBA2 recipes.
PICARD: add support for SCUBA2_CHECK_RMS

SCUBA2_CHECK_RMS sets the CHECK_RMS_DATA Frame uhdr - if this is
present then keep the cropped files if desired (KEEPFILES=1).

The code has been refactored to remove redundant statements.