Fix colors not being reset on accept w/ recent ZSH (fixes zsh-users#789)

The ZSH manual describes `region_highlight` as being an array in
https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html#Character-Highlighting,
therefore the previous strategy of removing as many characters as the
last suggestion is *not* the way to do it, explaining why it broke on
recent ZSH versions.

Replace this logic with a simple last-element delete. Keeps the
`_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT` variable intact since there's no
downside in tracking its content, as it still used as a marker for
whether a suggestion highlight was applied.

src/highlight.zsh

@@ -8,7 +8,7 @@ _zsh_autosuggest_highlight_reset() {
 	typeset -g _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 
 	if [[ -n "$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT" ]]; then
-		region_highlight=("${(@)region_highlight:#$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT}")
+		shift -p region_highlight
 		unset _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 	fi
 }

zsh-autosuggestions.zsh

@@ -244,7 +244,7 @@ _zsh_autosuggest_highlight_reset() {
 	typeset -g _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 
 	if [[ -n "$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT" ]]; then
-		region_highlight=("${(@)region_highlight:#$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT}")
+		shift -p region_highlight
 		unset _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 	fi
 }