docs: remove the 2.6 "Upgrading I2C Drivers" guide

All the drivers have long since been upgraded, and all the important
information here is also included in the "Implementing I2C device
drivers" guide.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20200806161456.8680-1-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>