Remove unnecessary information from private message classes

...iarproject/briar/android/contact/ContactListFragment.java

.../briarproject/briar/android/contact/ConversationItem.java

...briarproject/briar/android/forum/ForumControllerImpl.java

...ndroid/privategroup/conversation/GroupControllerImpl.java

...rg/briarproject/briar/api/blog/BlogInvitationRequest.java

...g/briarproject/briar/api/blog/BlogInvitationResponse.java

.../org/briarproject/briar/api/client/BaseMessageHeader.java

.../briarproject/briar/api/forum/ForumInvitationRequest.java

...briarproject/briar/api/forum/ForumInvitationResponse.java

...arproject/briar/api/introduction/IntroductionMessage.java

...arproject/briar/api/introduction/IntroductionRequest.java

...rproject/briar/api/introduction/IntroductionResponse.java

...riarproject/briar/api/messaging/PrivateMessageHeader.java

...r/api/privategroup/invitation/GroupInvitationRequest.java

.../api/privategroup/invitation/GroupInvitationResponse.java

...org/briarproject/briar/api/sharing/InvitationMessage.java

...org/briarproject/briar/api/sharing/InvitationRequest.java

...rg/briarproject/briar/api/sharing/InvitationResponse.java

...in/java/org/briarproject/briar/api/sharing/Shareable.java

...rproject/briar/introduction/IntroduceeProtocolEngine.java