Delete local bug database.

Written for bug-repo-syncer by Eike on 2012-04-03

The storage format has gradually changed over the course of the development.
It can therefore happen that bug-repo-syncer reads an old bug database
seemingly without errors, but it creates erroneous results when it processes
the bugs.

Therefore delete the local bug database when you upgrade to version 0.3.0.
The database is in the file "sync-data.pickle".

I have just encountered odd behavior in one of my other projects (Freeode).
The symptom is that all bug links show up like "bug #123@repo", while the
program can not translate any bug ID in the bug description. This also leads
to additional duplication of lines in the bug description.

The error is a bit tricky to spot, because there will always be some text
duplication when you upgrade to version 0.3.0: Version 0.2.0 creates
un-translatable bug IDs in the bug description. Attempts to translate them
leads to different results in each repository and causes duplication of lines
that contain bug IDs.