function removeKeyword(keyword) {
  /* jshint validthis: true */
  var RULES = this.RULES;
  delete RULES.keywords[keyword];
  delete RULES.all[keyword];
  delete RULES.custom[keyword];
  for (var i=0; i<RULES.length; i++) {
    var rules = RULES[i].rules;
    for (var j=0; j<rules.length; j++) {
      if (rules[j].keyword == keyword) {
        rules.splice(j, 1);
        break;
      }
    }
  }
  return this;
}
Remove keyword
@this  Ajv
@param {String} keyword pre-defined or custom keyword.
@return {Ajv} this for method chaining