Parse vars in classes the same as vars in other locations (#7188)

Class vars are still restricted to simple `name: type` bindings, not
full patterns. This is now handled in the check phase instead of during
parsing.

This is in preparation for supporting `static var`.

toolchain/check/node_stack.h

@@ -414,7 +414,6 @@ class NodeStack {
       -> std::optional<Id::Kind> {
     switch (node_kind) {
       case Parse::NodeKind::CallExprStart:
-      case Parse::NodeKind::FieldNameAndType:
       case Parse::NodeKind::IfExprThen:
       case Parse::NodeKind::RequireIntroducer:
       case Parse::NodeKind::ShortCircuitOperandAnd:
@@ -454,8 +453,6 @@ class NodeStack {
       case Parse::NodeKind::ClassIntroducer:
       case Parse::NodeKind::CodeBlockStart:
       case Parse::NodeKind::ExplicitParamListStart:
-      case Parse::NodeKind::FieldInitializer:
-      case Parse::NodeKind::FieldIntroducer:
       case Parse::NodeKind::ForHeaderStart:
       case Parse::NodeKind::FunctionIntroducer:
       case Parse::NodeKind::IfStatementElse: