remove `DISABLE_UPDATE_PROMPT=true` from instant prompt

If this breaks your shell, it means you are using an old version of
oh-my-zsh, which predates this commit:

fe0dd8226d

You need to upgrade oh-my-zsh by running the following command:

    omz update

internal/p10k.zsh

@@ -6515,7 +6515,6 @@ _p9k_dump_instant_prompt() {
   }
   zshexit_functions=(_p9k_instant_prompt_cleanup $zshexit_functions)
   precmd_functions=(_p9k_instant_prompt_precmd_first $precmd_functions)
-  DISABLE_UPDATE_PROMPT=true
 } && unsetopt prompt_cr prompt_sp && typeset -gi __p9k_instant_prompt_sourced='$__p9k_instant_prompt_version' ||
   typeset -gi __p9k_instant_prompt_sourced=${__p9k_instant_prompt_sourced:-0}'
     } always {