ref(utils): Remove unused to_timestamp helper and epoch constant (#8147)

### Description

Removes `to_timestamp()` and the module-level `epoch` constant from
`sentry_sdk/utils.py`. Nothing uses either of them.

#4899 (removing the old metrics code) deleted the last three callers of
`to_timestamp`, all in `sentry_sdk/metrics.py`, plus its import there.
`epoch` was only ever used inside `to_timestamp`.

**Evidence**
- `grep -rnw to_timestamp sentry_sdk tests scripts` only matches the
definition.
- `grep -rnw epoch sentry_sdk tests` only matches the definition and its
use in `to_timestamp`. The other match is the `(?P<epoch>...)` regex
group in the version parser, which isn't related.
- `git log -S "to_timestamp(" -- sentry_sdk tests` shows #4899 as the
last change to touch a call site.
- No open PR diff in this repo mentions `to_timestamp` or `epoch`.

**Why it's safe**
- `sentry_sdk.utils` is an internal module. Neither name is re-exported
from `sentry_sdk` or listed in `docs/`, and neither appears in
`CHANGELOG.md` or `MIGRATION_GUIDE.md`.
- The helper had a latent bug: it subtracted from a naive
`datetime(1970, 1, 1)`, so passing it a timezone-aware datetime (the SDK
uses aware UTC datetimes everywhere) raised `TypeError`. Leaving it
around invites someone to reuse it by mistake.
- `datetime` is still imported and used elsewhere in the module.
- No runtime behavior changes.

**Checks**
- `uv run ruff check sentry_sdk tests` and `ruff format --check` pass.
- `uv run --group typing mypy sentry_sdk` passes with no issues.
- `TESTPATH="tests/test_utils.py tests/test_basics.py
tests/test_client.py" uv run tox -e py3.14-common`: 587 passed, 3
skipped.

#### 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>

Author: sentry-junior[bot]

sentry_sdk/utils.py

@@ -79,8 +79,6 @@
     R = TypeVar("R")
 
 
-epoch = datetime(1970, 1, 1)
-
 # The logger is created here but initialized in the debug support module
 logger = logging.getLogger("sentry_sdk.errors")
 
@@ -248,10 +246,6 @@ def capture_internal_exception(exc_info: "ExcInfo") -> None:
         logger.error("Internal error in sentry_sdk", exc_info=exc_info)
 
 
-def to_timestamp(value: "datetime") -> float:
-    return (value - epoch).total_seconds()
-
-
 def format_timestamp(value: "datetime") -> str:
     """Formats a timestamp in RFC 3339 format.