Remove stale comment (#4727)

Comment became out of date with #4698 . Seems better to delete instead
of update this comment.

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/sem_ir/yaml_test.cpp

@@ -89,7 +89,6 @@ TEST(SemIRTest, YAML) {
       Pair("constant_values",
            Yaml::Mapping(AllOf(Each(Pair(inst_id, constant_id))))),
       Pair("symbolic_constants", Yaml::Mapping(SizeIs(0))),
-      // This production has only two instruction blocks.
       Pair("inst_blocks",
            Yaml::Mapping(ElementsAre(
                Pair("inst_block_empty", Yaml::Mapping(IsEmpty())),