Clean up .gitmodules a little

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
.gitmodules: really say good-bye to contrib

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Replace a few fiji.sc URLs with github.com ones in .gitmodules

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>