fix basename polyfill

lib/basename.fish

@@ -3,12 +3,9 @@
 #   basename [-s suffix] <string> [string...]
 #
 # OVERVIEW
-#   osx style variable arguments basename
+#   variable arguments basename
 
 function basename -d "get the filename or directory part of a path"
-  if test (uname) = "Darwin"
-    command basename $argv
-  else
   if set -q argv[1]
     set -l ext ""
     switch $argv[1]
@@ -25,5 +22,4 @@ function basename -d "get the filename or directory part of a path"
       command basename "$path" "$ext"
     end
   end
-  end
 end