Remove unnecessary self-insert override, close #20

@sunaku experimented and found that highlights are removed on
space bar and printable character key presses making the removed
code unnecessary.

See https://github.com/zsh-users/zsh-history-substring-search/issues/8
for more information.

plugins/history-substring-search/history-substring-search.zsh

@@ -96,18 +96,6 @@ if [[ $+functions[_zsh_highlight] -eq 0 ]]; then
     region_highlight=()
   }
 
-  #
-  # Remove existing highlights when the user
-  # inserts printable characters into $BUFFER
-  #
-  function ordinary-key-press() {
-    if [[ $KEYS == [[:print:]] ]]; then
-      region_highlight=()
-    fi
-    zle .self-insert
-  }
-  zle -N self-insert ordinary-key-press
-
   #
   # The following snippet was taken from the zsh-syntax-highlighting project:
   #