[Curl] Remove unnecessary member from NetworkStorageSession.

[Curl] Remove unnecessary member from NetworkStorageSession.
https://bugs.webkit.org/show_bug.cgi?id=194137

Patch by Takashi Komori <Takashi.Komori@sony.com> on 2019-01-31
Reviewed by Don Olmstead.

Source/WebCore:

No new tests since there is no behavior change.

* platform/network/NetworkStorageSession.h:
* platform/network/curl/NetworkStorageSessionCurl.cpp:
(WebCore::NetworkStorageSession::NetworkStorageSession):
(WebCore::NetworkStorageSession::context const): Deleted.

Source/WebKit:

* NetworkProcess/curl/NetworkProcessCurl.cpp:
(WebKit::NetworkProcess::platformCreateDefaultStorageSession const):

Source/WebKitLegacy:

* WebCoreSupport/NetworkStorageSessionMap.cpp:
(NetworkStorageSessionMap::defaultStorageSession):
(NetworkStorageSessionMap::ensureSession):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@240835 268f45cc-cd09-0410-ab3c-d52691b4dbfc