test: remove unused var from test-assert.js

`aethrow` is defined as a function returned from makeBlock() but the
function is never used and the call to makeBlock() does not run any
tests.

PR-URL: nodejs#4405
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-assert.js

@@ -271,8 +271,6 @@ assert.throws(makeBlock(a.deepStrictEqual, new Boolean(true), {}),
 function thrower(errorConstructor) {
   throw new errorConstructor('test');
 }
-var aethrow = makeBlock(thrower, a.AssertionError);
-aethrow = makeBlock(thrower, a.AssertionError);
 
 // the basic calls work
 assert.throws(makeBlock(thrower, a.AssertionError),