Remove unused "override comment" from ConditionTrait implementation

[stmontgomery]

This removes a mechanism in the implementation of ConditionTrait for a condition closure to provide an "override" comment.

Motivation:

While reviewing a draft evolution proposal for ConditionTrait (swiftlang/swift-evolution#2740) I realized that we don't use this "comment override" mechanism anywhere in the testing library. I believe we did back when it was first added, but it's no longer used.

Removing this would allow the public API being proposed above to be simplified: the evaluate() method could return Bool instead of a tuple.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[stmontgomery]

Related to #903

cc @Uncommon