GeezerVison

### Description

Removes the module-level `base64_stripped` regex string from
`sentry_sdk/tracing_utils.py`. Nothing has used it since 2023.

#1179 (Sep 2021) added it to validate the base64-encoded Sentry entry in
the `tracestate` header. #1907 (Feb 2023, "Remove deprecated
`tracestate`") deleted its only use,
`re.search("^{b64}$".format(b64=base64_stripped), sentry_value)`, but
left the definition and its comment behind.

**Evidence**
- `git grep -n base64_stripped` across the whole repo matches only the
definition.
- `git log -S base64_stripped` shows exactly two commits: the one that
added it (#1179) and the one that removed its only caller (#1907).
- No open PR or issue in this repo mentions `base64_stripped`.

**Why it's safe**
- It's a plain string constant in an internal module. It's not
re-exported from `sentry_sdk` and doesn't appear in `docs/`,
`CHANGELOG.md` or `MIGRATION_GUIDE.md`.
- It was never compiled or read at runtime, so behavior doesn't change.
- `re` is still used by `SENTRY_TRACE_REGEX` and other code in the
module.

**Checks**
- `uv run ruff check` and `ruff format --check` on the file pass.
- `uv run --group typing mypy sentry_sdk` passes with no issues.
- `TESTPATH=tests/tracing uv run tox -e py3.14-common`: 426 passed.

#### Issues
None. Found by a dead-code sweep.

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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>