Cleanup: remove the `just_used_solve' field from a number of games

Cleanup: remove the `just_used_solve' field from a number of games
which didn't actually need it. It was originally introduced in
Fifteen to suppress animation on Solve moves, but midend.c now does
that centrally unless the game specifically instructs it otherwise.
Therefore, just_used_solve is obsolete in all games which previously
used it. (Mines was even worse: it scrupulously maintained the
correctness of the field but never used it!)

Untangle is exempt from this cleanup: its `just_solved' field is
used to change the _length_ of the animation on Solve moves, not to
suppress it entirely, and so it has to stay.

[originally from svn r6419]