i2c: Remove I2C_COMPAT config symbol and related code

This code was added with 2bb5095a ("i2c: Provide compatibility links
for i2c adapters"). Commit message stated: Provide compatibility links
for [...] the time being. We will remove them after a long transition
period.
15 years should have been a long enough transition period.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>