Consider removing `CROSSGEN_COMPILE` define and the CrossGen binary

[AaronRobinsonMSFT]

Now that CrossGen2 has replaced CrossGen, we should consider removing code guarded by the following defines:

• Remove code guarded by CROSSGEN_COMPILE #57657
• Remove code guarded by FEATURE_NATIVE_IMAGE_GENERATION #57658
• Remove code guarded by FEATURE_READYTORUN_COMPILER (outside of the coreclr/jit directory. It is still used there) #57659
• Remove code guarded by FEATURE_PREJIT #57660
• Remove code guarded by FEATURE_NGEN_RELOCS_OPTIMIZATIONS #57661
• Remove PlainPointer/RelativePointer/RelativeFixupPointer #57706

/cc @jkotas @mangod9 @davidwrighton

[mangod9]

@dotnet/crossgen-contrib . Thanks for creating the issue, its been on our radar to first remove crossgen from building to determine what things break :)

[jkoritzinsky]

Fixing this will also significantly increase the speed of the CoreCLR native build (as crossgen and its object/static libraries make up somewhere between 1/4 and 1/3 of the build as a whole)

[AaronRobinsonMSFT]

@jkoritzinsky Yep. That was my secondary realization. My first was the amount of code we could remove but I'd also assumed we would build faster - thanks for confirming that assumption!

@agocke I assume improving build speeds would be a win for you too 😄

[agocke]

Hell yeah