Remove class JabRefMain.

The class JabRefMain was just a wrapper or bootstrapper for JabRef's
psvm. I personally dislike catching exception in the psvm (where we
cannot solve anything) and checking for runtime conditions (like JDK
version). Thus I decided by plain removal of that file.
Remove 'FileLinksUpgradeWarning' and 'PushToWinEdt'.

I really do not care about upgrading from old file links to a newer
format. Regarding 'PushToWinEdt', my version of JabRef will only
support Open Source Software options. Nevertheless, WinEdt is not
as commonly found between our users anymore.
Clean up.