Simplify timing-adapter settings

The targets only have one NPU so simplify the timing adapters settings
by removing unnecessary nesting and complexity.

Corstone-310 does not have timing adapters in the NPU model, so remove
that dead timing adapter code completely.

Change-Id: Iabe95c5e6354f5168beefa3ddfff4a348c4a7021
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>