Small cleanups for LoweringContext and BackendImplInterface (#67791)
* Clean up LoweringContext
* Clean up BackendImplInterface
Remove device/host related APIs that aren't yet needed and should
be redesigned if/when needed to support distributed / XLA backend.
- GetResourceDomain was being hashed but it's always an empty string
so far; We may want to add back a way to handle this, but we should
design it holistically with however we end up supporting distributed
