Revert "[VM/runtime] Refactor the representation of type parameters in the VM."

This reverts commit 8a21ab195a.

Reason for revert: Test failure: http://b/187227619

Original change's description:
> [VM/runtime] Refactor the representation of type parameters in the VM.
>
> This introduces a new VM internal class 'TypeParameters' representing the declaration of a list of type parameters, either in a class or function.
> The reference to (or use of) a type parameter is still represented by the existing 'TypeParameter' class.
>
> Fixes https://github.com/dart-lang/sdk/issues/43901
> Fixes https://github.com/dart-lang/sdk/issues/45763
>
> TEST=existing ones and a regression test
>
> Change-Id: I1fde808bf753cc1cb829f2c4383c1836651cee80
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189942
> Commit-Queue: Régis Crelier <regis@google.com>
> Reviewed-by: Alexander Markov <alexmarkov@google.com>

TBR=rmacnak@google.com,alexmarkov@google.com,regis@google.com,sstrickl@google.com

Change-Id: If12caa1a84cb6d1c1b8225589f3c994d25abb120
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/198282
Reviewed-by: Michal Terepeta <michalt@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Michal Terepeta <michalt@google.com>

runtime/vm/raw_object_fields.cc

@@ -136,12 +136,9 @@ namespace dart {
   F(TypeRef, type_test_stub_)                                                  \
   F(TypeRef, type_)                                                            \
   F(TypeParameter, type_test_stub_)                                            \
+  F(TypeParameter, name_)                                                      \
   F(TypeParameter, hash_)                                                      \
   F(TypeParameter, bound_)                                                     \
-  F(TypeParameters, names_)                                                    \
-  F(TypeParameters, flags_)                                                    \
-  F(TypeParameters, bounds_)                                                   \
-  F(TypeParameters, defaults_)                                                 \
   F(Closure, instantiator_type_arguments_)                                     \
   F(Closure, function_type_arguments_)                                         \
   F(Closure, delayed_type_arguments_)                                          \