V4L/DVB (4960): Removal of unused code from usbvision-i2c.c

i2c_adap is almost not used. This patch removes it, cleaning the i2c support,
and improving driver understanding.
Thanks to Thierry Merle for testing it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>