Remove soong.config, FileConfigurableOptions, MegaDevice config.

No current use cases for soong.config and user-editable local
configuration options for Soong, so let's remove it to reduce the API
surface.

The MegaDevice configuration is used for building every module, for
every cpu variant, for every architecture, but it apparently isn't
used for a while now.

Test: m nothing
Test: TH
Fixes: 174188200
Change-Id: I2550e70ff6f9c8b57e9a7cc517d6a119a032a27a