[iOS][WebKit2] Remove JoinExistingSession from plist

[iOS][WebKit2] Remove JoinExistingSession from plist
https://bugs.webkit.org/show_bug.cgi?id=128318
<rdar://problem/15971612>

Reviewed by Tim Horton.

Remove JoinExistingSession key that is not available on iOS.

* NetworkProcess/EntryPoint/mac/XPCService/NetworkService.Development/Info-iOS.plist:
* NetworkProcess/EntryPoint/mac/XPCService/NetworkService/Info-iOS.plist:
* WebProcess/EntryPoint/mac/XPCService/WebContentService.Development/Info-iOS.plist:
* WebProcess/EntryPoint/mac/XPCService/WebContentService/Info-iOS.plist:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@163587 268f45cc-cd09-0410-ab3c-d52691b4dbfc