gpio: Remove obsolete API 1.0 callback mechanism

In 1.0 you could set only one callback on the whole gpio controller. It
was impossible for another sub-system to add another callback, without
overwritting an existing one.

Such API has been obsolete for a long time and no one is using it
anymore. Thus removing it entirely.

Change-Id: I6a17fd99373dc6cef1fa2ebb421e992412d5015e
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>