[v9]: Remove `enableTracing`

[buenaflor]

Description

### Tasks
- [ ] https://github.com/getsentry/sentry-dart/pull/2695
- [ ] https://github.com/getsentry/sentry-docs/pull/12561

[kahest]

re Update docs - this was already done in getsentry/sentry-docs#12561, there shouldn't be any leftovers I think

[buenaflor]

ah yes thx

the only thing left is to remove the usages in the code