Skeletal mesh ddc key is always regenerated on load when LOD bones to remove are set

In FMeshBoneReduction::ReduceBoneCounts we are calling InvalidateDeriveDataCacheGUID.  If a user has specified bones in the Bones To Remove array for the mesh LOD, this will invalidate the guid.  The problem is that this code can be called when we are building for the DDC via USkeletalMesh::CacheDerivedData.  So we end up in a state where the DDC key is always regenerated.  As well as the repro above, you can also see the warning when loading Game Animation Sample.

We should do a pass on the DDC key generation for the mesh's source models, the LOD settings and the ref skeleton.  (A quick check of FSkeletalMeshLODInfo::ComputeDeriveDataCacheKey shows we aren't generating the key against all properties - eg. LODHysteresis).  Once we know the key generation is correct, the call to InvalidateDeriveDataCacheGUID can be removed.

1. Open a skeletal mesh and add some bones to the Bones To Remove array in the LOD Info
   1. Do this for LOD 0
2. Save the mesh
3. Close the editor and reload
4. You should see a warning in the log "Skeletal mesh [%s]: The derived data key is different after the build. Save the asset to avoid rebuilding it everytime the editor load it."
   1. An alternative to reopening the editor is to reload the asset from the content browser.  But for this to repro the issue you have to change FSkeletalMeshRenderData::Cache so we don't use the mesh from the ddc
      1. ie. the following if has to be false:

if(bTemp && bAllowDDCFetch && DDCUtils64Bit::GetSynchronous(DerivedDataKey, Owner, DerivedData))