Remove legacy migration prototype (#4887)

Although migration tooling is intended for Carbon, this tooling was
primarily a proof-of-concept prototype. The last time it got significant
work was in 2022
(https://github.com/carbon-language/carbon-lang/commits/trunk/migrate_cpp);
since then, we've mainly been doing small cleanups to keep it building.
We're now hitting an issue in #4886 that `TypeNodes.inc` isn't exposed
as a `#include`.

We already had thoughts about rewriting this as a `RecursiveASTVisitor`,
per `rewriter.cpp`/`rewriter.h`. That may justify a significantly
different approach than had been set up in `cpp_refactoring`. But, it's
hard to tell.

Either way, my sense is that rather than incrementally trying to keep
this code building, we should revisit it when we're ready and have a
long-term strategy for how migration should work.

Author: jonmeow

bazel/check_deps/BUILD

@@ -15,8 +15,6 @@ filegroup(
     data = [
         "//explorer",
         "//installers/local:carbon",
-        "//migrate_cpp:rewriter",
-        "//migrate_cpp/cpp_refactoring",
         "//toolchain/install:carbon-busybox",
         # The tree sitter rules can't be queried; evaluation fails on
         # @platforms.