media: cx25821: drop duplicated i2c_slave_did_ack()

This function is defined twice, one at cx25821-i2c and the other
inside cx25821-core. It turns that only the first
is actually used inside the code.

This causes a clang warning.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>