client: drop unused event queue cond and list variables

The wl_event_queue cond variable has been replaced by the wl_display
reader_cond variable (commit 3c7e8bfb).
This cond variable is never waited for anymore, just
signaled/broadcasted, and thus can be safely removed.

The wl_display event_queue_list and link from wl_event_queue
can be removed as well, since it was only used to iterate over
the event queue list in order to broadcast the now unused cond.

No regression on queue unit tests.

Signed-off-by: Olivier Blin <olivier.blin@softathome.com>

v2: fixed and rebased after 886b09c9
    added signed-off-by

v3: removed link from wl_event_queue

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>