fix(build): move no-unused-variables to tslint.build.json

In the latest version of tslint, the rule "no-unused-variables" requires
type information and therefore cannot be run on the fly from VS Code.

This commit move the configuration of "no-unused-variables"
from tsling.common.json to tslint.build.json to avoid warnings printed
to VS Code console:

    Warning: The 'no-unused-variable' rule requires type information.

packages/build/config/tslint.build.json

@@ -9,12 +9,12 @@
   "rules": {
     // These rules find errors related to TypeScript features.
 
-
     // These rules catch common errors in JS programming or otherwise
     // confusing constructs that are prone to producing bugs.
 
     "await-promise": true,
     "no-floating-promises": true,
+    "no-unused-variable": true,
     "no-void-expression": [true, "ignore-arrow-function-shorthand"]
   }
 }

packages/build/config/tslint.common.json

@@ -20,7 +20,6 @@
     "no-shadowed-variable": true,
     "no-string-throw": true,
     "no-unused-expression": true,
-    "no-unused-variable": true,
     "no-var-keyword": true,
     "triple-equals": [true, "allow-null-check", "allow-undefined-check"]
   }