Drop the filtering of :args and :argu from the session file.

I haven't experienced the bug that is hinted at in the comments, and I see the argument list as an essential part of the session information.
FIX: Determine tab / global scope from sourced session, not the config.

For the message after opening a session, use the t:this_session / v:this_session information from the loaded session instead of the current configuration. This matters especially when a session is auto-loaded on Vim startup, because then the scope cannot be determined from the command, (since there is no command).
The :OpenSession / :OpenTabSession commands actually perform the same (maybe there should be a warning when a global session is opened through :OpenTabSession?!)
Refactoring: Avoid duplicate conditional.
Allow to forget a saved default session, too.

Shutting off the query for auto-loading of a previous session is just as important for the default session, not just custom sessions. This was also the behavior of my original pull request; I don't know why you left that part out.
Merge branches 'drop-args-filtering', 'fix-session-scope-on-open' and 'allow-forgetting-default-session'