Runtime: Remove JIT LEGACY_BACKEND code

[BruceForstall]

Remove the obsolete JIT LEGACY_BACKEND code.

This was only ever supported for x86 and arm32. The x86 LEGACY_BACKEND path has not been built for tested for several months. For arm32, LEGACY_BACKEND is currently still built and can be tested, but RyuJIT/arm32 (the non-LEGACY_BACKEND code) has been the default for many months.

Also, delete the legacy backend support in netci.groovy, in the desktop build, and other places.

This will _drastically_ simplify the JIT source code, and will complete our multi-year transition to the RyuJIT architecture.

[BruceForstall]

@dotnet/jit-contrib @dotnet/arm32-contrib @dotnet/arm64-contrib I plan to start this work around May 15.

[BruceForstall]

Fixed by dotnet/coreclr#18064