sequencer.c: split up sequencer_remove_state()

sequencer.c: split up sequencer_remove_state()

Split off the free()-ing in sequencer_remove_state() into a utility
function, which will be adjusted and called independent of the other
code in sequencer_remove_state() in a subsequent commit.

The only functional change here is changing the "int" to a "size_t",
which is the correct type, as "xopts_nr" is a "size_t".

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>