Tidy up core games

[ace-dent]

Avoid unused imports and remove some dead code.

• Remove unused imports, e.g. uos,utime,framebuf, etc.
• Avoid machine import; just import freq as needed and use thumby.reset().
• Format frequency for readability x_000_000.
• Remove dead code from 'Thumgeon' and 'SpaceDebris'.
• Trailing whitespace cleanup.

[ace-dent]

@TinyCircuits - Please also include these commits in cutting a new firmware release. Thanks.