refactor(rtx): remove deprecated `rtx` plugin in favor of `mise`

plugins/mise/mise.plugin.zsh

@@ -24,3 +24,4 @@ fi
 
 # Generate and load mise completion
 $__mise completion zsh >| "$ZSH_CACHE_DIR/completions/_$__mise" &|
+unset __mise

plugins/rtx/rtx.plugin.zsh

@@ -1,2 +0,0 @@
-# TODO: 2024-01-03 remove rtx support
-echo "[oh-my-zsh] 'rtx' plugin has been renamed to 'mise'"