befs: remove unused BEFS_BT_PARMATCH

befs_btree_find(), the only caller of befs_find_key(), only cares about if
the return from that function is BEFS_BT_MATCH or not. It never uses the
partial match given with BEFS_BT_PARMATCH. Make the overflow return clearer
by having BEFS_BT_OVERFLOW instead of BEFS_BT_PARMATCH.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Salah Triki <salah.triki@gmail.com>