code: 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;
  }
docs: Remove keyword
  @this  Ajv
  @param {String} keyword pre-defined or custom keyword.
  @return {Ajv} this for method chaining