Remove pattern evaluation from the interpreter. (#2400)

Patterns all compute their values when type-checked, so we never
actually need to do any multi-step evaluation to compute the value of a
pattern. Doing so was leading to quadratic runtime and excess noise in
the trace file.

explorer/interpreter/action_stack.cpp

@@ -144,7 +144,6 @@ static auto FinishActionKindFor(Action::Kind kind) -> FinishActionKind {
     case Action::Kind::ExpressionAction:
     case Action::Kind::WitnessAction:
     case Action::Kind::LValAction:
-    case Action::Kind::PatternAction:
       return FinishActionKind::Value;
     case Action::Kind::StatementAction:
     case Action::Kind::DeclarationAction: