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

I've been using the plugin for quite some time now (mostly on Windows/x64 with a rather recent Vim 7.3.823), and 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.

Please consider re-enabling this default functionality of `:mksession` (or at least provide a configuration to enable this, maybe marked as "experimental feature").