prompt: Remove 'powerlevel9k' deprecation warning

'powerlevel9k' is unsupported and deprecated for 2 years now. It's time
to cleanu the deprecation warning message.

modules/prompt/init.zsh

@@ -10,18 +10,8 @@ autoload -Uz promptinit && promptinit
 
 # Load the prompt theme.
 zstyle -a ':prezto:module:prompt' theme 'prompt_argv'
-if [[ "$TERM" == (dumb|linux|*bsd*) ]] || (( $#prompt_argv < 1 )); then
+if [[ $TERM == (dumb|linux|*bsd*) ]] || (( $#prompt_argv < 1 )); then
   prompt 'off'
-elif [[ "$prompt_argv[1]" == 'powerlevel9k' ]] ; then
-  <<EOW
-WARNING: Prezto does not support 'powerlevel9k' anymore as it has
-         been deprecated and is not supported by its author.
-         Consider migrating to 'powerlevel10k' instead by setting:
-         zstyle ':prezto:module:prompt' theme 'powerlevel10k'
-         in ${${ZDOTDIR:-$HOME}/#$HOME/~}/.zpreztorc.
-         Switching to prezto default prompt 'sorin'..."
-EOW
-  prompt 'sorin'
 else
   prompt "$prompt_argv[@]"
 fi