Remove unnecessary namespace and fix importing expression in test module

This implementation is for making `import * as someModule from 'someModule'` is available.
But this usage is note appropiate.

types/wcwidth/index.d.ts

@@ -5,6 +5,4 @@
 
 declare function wcwidth(input: string): number;
 
-declare namespace wcwidth {}
-
 export = wcwidth;

types/wcwidth/wcwidth-tests.ts

@@ -1,4 +1,4 @@
-import * as wcwidth from 'wcwidth';
+import wcwidth = require('wcwidth');
 
 const length1 = wcwidth('한');   // => 2