[build][intel-i2c] Remove Wno-unused-function suppression

This change removes the unused-function suppression from the Intel I2C
driver and cleans out `intel-i2c-driver` from the `Wno-unused-function`
config in `build/config/BUILD.gn`. Built the driver to verify there are
no remaining unused functions.

Bug: 42136088
Test: fx build with `fx add-test //src/devices/i2c/drivers/intel-i2c:package`
Change-Id: I80c12256ff728c5e4c88cd4f67710f7b9eb231ba
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1547483
Commit-Queue: John Wittrock <wittrock@google.com>
Reviewed-by: Braden Kell <bradenkell@google.com>
Reviewed-by: Prabhu Karthikeyan Rajasekaran <prabhukr@google.com>