media: imx274: remote unused function imx274_read_reg

imx274_read_reg() is not used since commit ca017467 ("media:
imx274: add helper to read multibyte registers").

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>