test: only include http module once

A few tests in test/gc include the http module twice. Remove duplicate
require().

PR-URL: https://github.com/nodejs/node/pull/4606
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/gc/test-http-client-connaborted.js

@@ -18,7 +18,6 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var http = require('http');
 var server = http.createServer(serverHandler);
 server.listen(PORT, getall);
 

test/gc/test-http-client-onerror.js

@@ -20,7 +20,6 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var http = require('http');
 var server = http.createServer(serverHandler);
 server.listen(PORT, runTest);
 

test/gc/test-http-client-timeout.js

@@ -22,7 +22,6 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var http = require('http');
 var server = http.createServer(serverHandler);
 server.listen(PORT, getall);
 

test/gc/test-http-client.js

@@ -18,7 +18,6 @@ let countGC = 0;
 
 console.log('We should do ' + todo + ' requests');
 
-var http = require('http');
 var server = http.createServer(serverHandler);
 server.listen(PORT, getall);