Don't use legacy SMP when GD is enabled

SMP channel should be used by GD SMP only.
Legacy SMP should not be initialized, and its APIs should not be used.

Tag: #gd-refactor
Bug: 158861440
Test: cert/run --host
Test: nRF app. Pair a remote device. Code path should be GD only.
Change-Id: I03e509d347dc0a4a8c62afd1e6af3eda033648db