i2c: core: Remove definition of i2c_smbus_process_call

i2c_smbus_process_call has no users in the kernel, so this can be
removed. Documentation for the same has been updated accordingly.

Fixes following sparse warning.
drivers/i2c/i2c-core.c:1871:5: warning: symbol 'i2c_smbus_process_call'
was not declared. Should it be static?

[wsa: updated the documentation]

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>