[wasm] Remove the in-repo Crossgen2Tasks shim for browser composite R2R once the SDK names wasm R2R outputs #135023

Browser composite ReadyToRun (PublishReadyToRunComposite=true, #134618) needs ReadyToRun SDK tasks that name the composite owner <entry>.r2r.wasm and write each component stub as <name>.wasm. The base SDK's tasks don't do this yet; that's dotnet/sdk#56395. Until it flows into the SDK that dotnet/runtime builds and tests with, the in-repo src/tasks/Crossgen2Tasks shim stands in for it.

When an SDK with wasm ReadyToRun output naming is in use, remove:

• src/libraries/sendtohelix-browser.targets: the '$(Scenario)' == 'BuildWasmApps' clause on the Crossgen2Tasks HelixCorrelationPayload (Wasm.Build.Tests don't need the shim on Helix anymore).
• src/mono/wasm/Wasm.Build.Tests/ReadyToRunTests.cs: the composite-only Crossgen2SdkOverridePropsPath/Crossgen2SdkOverrideTargetsPath wiring in GetR2RBuildArgs, so composite also runs on the stock SDK tasks.
• src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.CoreCLR.targets: _WasmCoreClrValidateCompositeTasks, which fails fast when the tasks plan a non-.r2r.wasm owner. Remove it, or keep it only as a guard against older SDKs.

Related: the WASI owner name now matches browser (#134874), and dotnet/sdk#56395 needs the matching change (dotnet/sdk#56395 (comment)).