IB/uverbs: Remove unnecessary list_del()s

In ib_uverbs_cleanup_ucontext(), when iterating through the lists of
objects, there's no reason to do list_del() to remove the objects,
since both the objects and the lists that contain them are about to be
freed anyway.  Since list_del() is a moderately big inline function,
getting rid of this extra work saves quite a bit of .text:

add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-217 (-214)
function                                     old     new   delta
ib_uverbs_comp_handler                       225     228      +3
ib_uverbs_async_handler                      256     255      -1
ib_uverbs_close                              905     689    -216

Signed-off-by: Roland Dreier <rolandd@cisco.com>