feat(frontends/lean): `lean --server` should display meta-variables using the approach used in check command, closes #280
feat(frontends/lean/builtin_cmds): remove workaround for getting nice metavariable names in the check command

We don't need it anymore after previous commit 2a16050
feat(frontends/lean): display metavariable application arguments in check command

The idea is to "fix" counter-intuitive output like the ones were
produced in the tests check.lean and check2.lean