Bazel 7 cleanup and dependency-wiring simplification

[psamanoelton]

Summary

This PR does a small round of Bazel 7 cleanup and dependency-wiring simplification on top of the already-working Bazel 7 / protobuf 6 baseline.

The goal is not to change dependency versions or CI behavior, but to reduce migration-specific scaffolding where it is no longer needed, keep the remaining shims clearly documented, and preserve a clean, working build/test state.

What changed

Repository helper cleanup

• simplified third_party/repo.bzl
• removed unused tb_http_archive support for build_file / link_files
• kept tb_http_archive focused on the behaviors still used in this repo:
  • mirrored URLs
  • optional patch application

WORKSPACE cleanup

• switched mirror-only repositories back to plain http_archive where the TensorBoard-specific helper was no longer adding value
• kept tb_http_archive(...) only for repositories that still need:
  • patch application
  • repo mapping
• clarified comments around the remaining Bazel/protobuf compatibility repos

Compatibility shim review

• verified that third_party/compatibility_proxy is still required by the current rules_java / protobuf loading path
• kept it in place rather than removing a live dependency
• verified that third_party/protobuf_pip_deps and third_party/protobuf_pip_deps_setuptools are still required by protobuf 6.31.1’s Bazel macros and Python packaging flow
• improved documentation explaining why those repos still exist

Why this PR is useful

This PR keeps the Bazel 7 migration easier to maintain by:

• removing dead helper code
• narrowing custom repository logic to the cases that still need it
• documenting the remaining compatibility shims with concrete rationale
• avoiding behavior changes to the working build graph

Validation

Validated with:

• bazel fetch //tensorboard/...
• bazel build //tensorboard/... --nobuild
• bazel build //tensorboard/...
• bazel test //tensorboard/plugins/debugger_v2:debugger_v2_plugin_test
• full local Bazel test runs after the cleanup changes