Clean up NETWORK_SESSION code after r191848

Clean up NETWORK_SESSION code after r191848
https://bugs.webkit.org/show_bug.cgi?id=150829

Patch by Alex Christensen <achristensen@webkit.org> on 2015-11-03
Reviewed by Darin Adler.

* NetworkProcess/NetworkResourceLoader.cpp:
(WebKit::NetworkResourceLoader::didReceiveData):
(WebKit::NetworkResourceLoader::didCompleteWithError):
* NetworkProcess/NetworkSession.h:
* NetworkProcess/cocoa/NetworkSessionCocoa.mm:
(WebKit::NetworkSession::NetworkSession):
(WebKit::NetworkSession::createDataTaskWithRequest):
(WebKit::NetworkSession::dataTaskForIdentifier):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191942 268f45cc-cd09-0410-ab3c-d52691b4dbfc