This suppressed any output when using RBENV_DEBUG=1 and does not really
hurt to have in the unlikely case that it should fail; you would get
two error messages now:
rbenv: ruby: command not found
rbenv: system version not found in PATH
On other systems, we expected to find system Ruby in `/usr/bin`, but in
OpenBSD 5.4 it will be found in `/usr/local/bin`.
This replaces the limited USRBIN_ALT hack with a more generic
`path_without` function that will ensure that the given executable is
not present in the resulting PATH even if it's found in multiple
system paths.