ref(sessions): Remove unused TERMINAL_SESSION_STATES constant (#8151)

### Description

Removes the module-level `TERMINAL_SESSION_STATES` tuple from
`sentry_sdk/sessions.py`. It has never been read anywhere.

#627 (Mar 2020) added it with the original session tracking code, and
#985 (Feb 2021) moved it a few lines down. Neither change, nor any other
commit, added a use of it. Session status handling (`Session.close()`,
`SessionFlusher` aggregation) compares against the status literals
directly.

**Evidence**
- `git grep TERMINAL_SESSION_STATES` across the whole repo matches only
the definition.
- `git log -G TERMINAL_SESSION_STATES` shows only #627 (added) and #985
(moved).
- No open PR or issue mentions it.

**Why it's safe**
- It's a plain 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 read at runtime, so behavior doesn't change.

#### Issues
n/a

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via **Ivana Kellyer**.
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Ivana Kellyer <ivana.kellyer@sentry.io>

Author: sentry-junior[bot]

sentry_sdk/sessions.py

@@ -132,7 +132,6 @@ def track_session(
             scope.end_session()
 
 
-TERMINAL_SESSION_STATES = ("exited", "abnormal", "crashed")
 MAX_ENVELOPE_ITEMS = 100