[wasm-mt] Settle on MonoWasmBuildVariant to control build variants #75152
Description
Instead of WasmEnableThreads=true (resp WasmEnablePerfTracing=true) just use MonoWasmBuildVariant=multithread (resp MonoWasmBuildVariant=perftrace) everywhere. I'm not sure why we need two mechanisms for doing the same thing.