Cleanup targets.py

[felker]

• Consider renaming TTDTarget to LogTTDTarget for clarity
• 7x threshold_range() class methods are never called anywhere. Vestige of old ROC calculation, e.g. in https://github.com/PPPLDeepLearning/plasma-python/blob/v0.9/plasma/utils/performance.py
• MaxHingeTarget.loss() (method unique to that class) is unused
• Base class Target is an invalid configuration option (predicts TTD directly, without the 0 mask before T_warning like in TTDLinearTarget?)

Related to #53, and the broader efforts of #42.

[felker]

Related: eb8f664