opam switch remove should offer to clean up a stale directory

[dra27]

Somewhat related to #1636 and friends.

The messages from opam here are confusing:

C:\Devel\opam-repository5>opam switch create ocaml-5.1.1-compressed ocaml-variants.5.1.1+options
[ERROR] Directory "C:\\Users\\DRA\\AppData\\Local\\opam\\ocaml-5.1.1-compressed" already exists, please choose a different name

C:\Devel\opam-repository5>opam switch remove ocaml-5.1.1-compressed
The compiler switch ocaml-5.1.1-compressed does not exist.

The cause is a leftover directory from an aborted switch. It would be nice if we changed opam switch remove to at least offer to remove the directory; it would be really nice if opam switch create offered similarly.