[prompt] Simplify short_pwd function

We don't need a second variable.

modules/prompt/functions/short_pwd

@@ -1,13 +1,10 @@
 # shortens the pwd for use in prompt
 
-local current_dir="${1:-${PWD}}"
-local return_dir='~'
-
-current_dir="${current_dir/#${HOME}/~}"
+local current_dir="${${1:-${PWD}}/#${HOME}/~}"
 
 # if we aren't in ~
 if [[ ${current_dir} != '~' ]]; then
-  return_dir="${${${${(@j:/:M)${(@s:/:)current_dir}##.#?}:h}%/}//\%/%%}/${${current_dir:t}//\%/%%}"
+  current_dir="${${${${(@j:/:M)${(@s:/:)current_dir}##.#?}:h}%/}//\%/%%}/${${current_dir:t}//\%/%%}"
 fi
 
-print ${return_dir}
+print ${current_dir}