Remove I18n._()

This function was already deprecated and still triggers a java warning.
Removing it silences that.

Author: matthijskooijman

arduino-core/src/processing/app/I18n.java

@@ -70,11 +70,6 @@ static protected void init(String language) throws MissingResourceException {
     PROMPT_BROWSE = tr("Browse");
   }
 
-  @Deprecated
-  public static String _(String s) {
-    return tr(s);
-  }
-
   public static String tr(String s) {
     String res;
     try {