tslint: no-unused-variable is deprecated.

Since TypeScript 2.9.  We are already using noUnusedLocals in
tsconfig.json.

tslint.json

@@ -40,7 +40,6 @@
     "no-require-imports": true,
     "no-string-throw": true,
     "no-unused-expression": true,
-    "no-unused-variable": true,
     "no-var-keyword": true,
     "object-literal-shorthand": true,
     "only-arrow-functions": [