Remove legacy support for MCG scenarios

[AaronRobinsonMSFT]

When there are no more uses of MCG generated source on CoreCLR, the following code paths should be removed.

• NativeCallableInternalAttribute - Legacy implementation for NativeCallableAttribute. See Accept NativeCallableInternalAttribute to support legacy scenario #37621. Note that CrossGen2 doesn't have support for MCG generated assemblies.
• ICastable - Most uses are under FEATURE_ICASTABLE.

[linuxuser27]

/cc @agocke