staging/easycap: kill EASYCAP_IS_VIDEODEV_CLIENT compilation conditional

remove EASYCAP_IS_VIDEODEV_CLIENT and irrelevant code as the define
is always set in the in-kernel driver

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>