Remove games_scores_backend_startup()
Remove a couple obsolete FIXME comments
Fix games_scores_backend_new()

I got very confused here when working on bda58eba77. This has nothing to
do with the player's name.
Set a name for the one GSource we create
trivial: use G_SOURCE_REMOVE instead of FALSE
Fix import of scores from 3.12 C games

This commit allows us to reuse scores from Five-or-more, Nibbles,
Robots, and Tali 3.12. These games saved scores without the player name.
Instead of discarding such scores as malformed, assume the scores were
earned by the current user.