assert: remove unnecessary use of __proto__

AssertionError already inherits from Error above using util.inherits(),
so this extra line was redundant.

test/simple/test-assert.js already tests for `instanceof`, and still passes.

lib/assert.js

@@ -49,6 +49,8 @@ assert.AssertionError = function AssertionError(options) {
     Error.captureStackTrace(this, stackStartFunction);
   }
 };
+
+// assert.AssertionError instanceof Error
 util.inherits(assert.AssertionError, Error);
 
 function replacer(key, value) {
@@ -85,10 +87,6 @@ assert.AssertionError.prototype.toString = function() {
   }
 };
 
-// assert.AssertionError instanceof Error
-
-assert.AssertionError.__proto__ = Error.prototype;
-
 // At present only the three keys mentioned above are used and
 // understood by the spec. Implementations or sub modules can pass
 // other keys to the AssertionError's constructor - they will be