[media] em28xx: get rid of the dummy clock source

The v4l2 dummy clock has been added with commit fc5d0f8a
("V4L2: em28xx: register a V4L2 clock source") to be able to use the ov2640
soc_camera driver.
Since commit 46796cfc ("ov2640: use standard clk and enable it") it is
no longer required.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>