Remove last references to sequencer

This needs a corresponding change to sequencer, namely adding the following
as a CONFIG_SNCSEQ file:
```make
ifdef T_A

DBDFILES += $(patsubst %.st,%_snl.dbd,$(notdir $(filter %.st,${SRCS})))
DBDFILES += $(patsubst %.stt,%_snl.dbd,$(notdir $(filter %.stt,${SRCS})))

endif
```

Note that we modify `DBDFILES` which is a bit of a "private variable" instead
of the more ideal `DBDS`; however, I really don't see a way to get around that
given that we only read the dependency information on pass three of this cursed
file, long after `DBDS` has been forgotten.