staging/easycap: Kill leftover build options in readme

The following options were removed from in-tree driver, but were left
in README:

- EASYCAP_IS_VIDEODEV_CLIENT was removed in cb81fa07 (staging/easycap:
  kill EASYCAP_IS_VIDEODEV_CLIENT compilation conditional);

- EASYCAP_NEEDS_V4L2_DEVICE_H/EASYCAP_NEEDS_V4L2_FOPS were removed in
  30516058 (staging/easycap: kill EASYCAP_NEEDS_V4L2_DEVICE_H and
  EASYCAP_NEEDS_V4L2_FOPS);

- EASYCAP_NEEDS_UNLOCKED_IOCTL was removed in f2b3c685 (staging/easycap:
  kill EASYCAP_NEEDS_UNLOCKED_IOCTL).

Remove them.

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>