[media] media: i2c: adv7604: Remove some unused functions

Removes some functions that are not used anywhere:
test_read() edid_read_block() dpp_write() dpp_read()
esdp_write() esdp_read() cec_write_clr_set()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>