Fix emulating the scenario where system Ruby is missing on OpenBSD

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.

test/prefix.bats

@@ -25,15 +25,6 @@ load test_helper
 }
 
 @test "prefix for invalid system" {
-  USRBIN_ALT="${RBENV_TEST_DIR}/usr-bin-alt"
-  mkdir -p "$USRBIN_ALT"
-  for util in head readlink greadlink; do
-    if [ -x "/usr/bin/$util" ]; then
-      ln -s "/usr/bin/$util" "${USRBIN_ALT}/$util"
-    fi
-  done
-  PATH_WITHOUT_RUBY="${PATH/\/usr\/bin:/$USRBIN_ALT:}"
-
-  PATH="$PATH_WITHOUT_RUBY" run rbenv-prefix system
+  PATH="$(path_without ruby)" run rbenv-prefix system
   assert_failure "rbenv: system version not found in PATH"
 }