remove repl-vm; repl evaluation does not cause recursive vm invocation

remove repl-vm; repl evaluation does not cause recursive vm invocation

* module/system/repl/common.scm (<repl>): Remove "vm" field and repl-vm
  accessor. I think the correct model is to just use the-vm. This change
  was prompted by the need to have the REPL itself not cause a recursive
  VM invocation, so that captured prompts at the REPL are rewindable.
  (make-repl): Remove treatment of #:vm.
  (repl-eval): Load a compiled expression as a simple thunk, avoiding a
  recursive VM call.

* module/system/repl/command.scm (profile, trace): Remove repl-vm
  treatment.
  (backtrace, debugger, step): Remove, as they were not implemented.