MINOR: lb/api: remove the locked argument from take_conn/drop_conn

MINOR: lb/api: remove the locked argument from take_conn/drop_conn

This essentially reverts commit 2b4370078 ("MINOR: lb/api: let callers
of take_conn/drop_conn tell if they have the lock") that was merged
during 2.4 before the various locks could be eliminated at the lower
layers. Passing that information complicates the cleanup of the queuing
code and it's become useless.