avcodec: v4l2_m2m: remove unnecessary timeout.

Qualcomm's db410c/db820 Venus driver currently present in mainline
kernel has a bug which mishandles the CMD_STOP requests causing the
decoder to block while draining [1].

This patch removes the workaround that was used to prevent that
situation.

Encoding/Decoding tested on db820c.

[1] on CMD_STOP, the driver is flushing all buffers and never raising
IPIPE which ends up in blocking on poll.
