[WebNN] Remove workarounds for TFLite backend (#23406)
The WebNN CPU device type may now target different backends, such as
CoreML. Legacy special workarounds for the TFLite backend should be
removed and allowed to fail as is, as these are implementation issues.
Additionally, the WebNN EP should adhere to the WebNN API conformance.
We assume all the WebNN ops should be supported, so remove the WebNN op
support status for different device types in webnn-operators.md as well.
