media: v4l: async: Fix duplicated list deletion

The list deletion call dropped here is already called from the
helper function in the line before. Having a second list_del()
call results in either a warning (with CONFIG_DEBUG_LIST=y):

list_del corruption, c46c8198->next is LIST_POISON1 (00000100)

If CONFIG_DEBUG_LIST is disabled the operation results in a
kernel error due to NULL pointer dereference.

Fixes: 28a12957 ("media: v4l: async: Allow multiple connections between entities")
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>