build: remove tslint workaround for custom rules (#19482)

Cleans up a workaround that was added to allow for our custom rules to be compiled while we were on the TS 3.9 RC version.

Author: crisbeto

tools/tslint-rules/tsLoaderRule.js

@@ -3,7 +3,7 @@ const Lint = require('tslint');
 
 // Custom rule that registers all of the custom rules, written in TypeScript, with ts-node.
 // This is necessary, because `tslint` and IDEs won't execute any rules that aren't in a .js file.
-require('ts-node').register({project: path.join(__dirname, './tsconfig.json')});
+require('ts-node').register({project: path.join(__dirname, '../gulp/tsconfig.json')});
 
 // Add a noop rule so tslint doesn't complain.
 exports.Rule = class Rule extends Lint.Rules.AbstractRule {