robbyrussell: remove local variable (#8131)

Now it can be sourced in a function, like zplug.

themes/robbyrussell.zsh-theme

@@ -1,5 +1,5 @@
-local ret_status="%(?:%{$fg_bold[green]%}➜ :%{$fg_bold[red]%}➜ )"
-PROMPT='${ret_status} %{$fg[cyan]%}%c%{$reset_color%} $(git_prompt_info)'
+PROMPT="%(?:%{$fg_bold[green]%}➜:%{$fg_bold[red]%}➜)"
+PROMPT+=' %{$fg[cyan]%}%c%{$reset_color%} $(git_prompt_info)'
 
 ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg_bold[blue]%}git:(%{$fg[red]%}"
 ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%} "