vfs/shell: remote unlink should report and handle errors

[tuffnatty]

Proposed changes

Drop -f option from rm invocation. Otherwise, an attempt to delete files in a write-protected remote location reports no error and keeps the files. It's a problem c313396 tried to solve but apparently didn't succeed.

I would have added a test but I'll wait till (if) #5157 with the test infrastructure is merged in.

• Fixes: failure while deleting is not reported when working over fish #1501

Checklist

• I have signed-off my contribution with git commit --amend -s
• Lint and unit tests pass locally with my changes (make indent && make check)
• I have added tests that prove my fix is effective or that my feature works

[tuffnatty]

Found and linked the erroneously closed issue #1501.

[zyv]

Looks reasonable to me. I didn't test it, though.