[mono] Clean up the delegate ctor code. (#83288)

[mono] Clean up the delegate ctor code. (#83288)

* [mono] Clean up the delegate ctor code.

* Merge the virtual and non-virtual cases.
* Reorder the code in handle_delegate_ctor () to reduce the llvm_only conditionals.
* Name variables etc. delegate info instead of delegate trampoline.
* Avoid setting the del->is_virtual field if its FALSE.

* Remove some code which is already done by mono_resolve_patch_target_ext ().

* Remove some dead code.

* Unify the virtual/non-virtual case for llvmonly as well.