merge: remove drop_save() in favor of remove_merge_branch_state()

merge: remove drop_save() in favor of remove_merge_branch_state()

Both remove_branch_state() and drop_save() delete almost the same set of
files about the current merge state. The only difference is MERGE_RR but
it should also be cleaned up after a successful merge, which is what
drop_save() is for.

Make a new function that deletes all merge-related state files and use
it instead of drop_save(). This function will also be used in the next
patch that introduces --quit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>