remove deprecated _append_path function

functions/_append_path.fish

-# Appends the path to the specified path list. If no list specified,
-#  defaults to $PATH
-function _append_path
-  set_color red
-  echo '_append_path function deprecated. Rename to _prepend_path.' >&2
-  set_color normal
-
-  set -l path PATH
-
-  if test (echo $argv | wc -w) -eq 2
-    set path $argv[2]
-  end
-
-  if test -d $argv[1]
-    if not contains $argv[1] $$path
-      set $path $argv[1] $$path
-    end
-  end
-end