ref: remove PREFER_CANONICAL_LEGACY_KEYS setting (always False)

[asottile-sentry]

introduced in https://github.com/getsentry/sentry/pull/8789 and has never been flipped

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.83%. Comparing base (861a9f9) to head (7295fde).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #70026 +/- ## ========================================== - Coverage 79.84% 79.83% -0.01% ========================================== Files 6500 6500 Lines 289071 289050 -21 Branches 49777 49776 -1 ========================================== - Hits 230798 230768 -30 - Misses 57915 57924 +9 Partials 358 358 ``` | [Files](https://app.codecov.io/gh/getsentry/sentry/pull/70026?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry) | Coverage Δ | | |---|---|---| | [src/sentry/conf/server.py](https://app.codecov.io/gh/getsentry/sentry/pull/70026?src=pr&el=tree&filepath=src%2Fsentry%2Fconf%2Fserver.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry#diff-c3JjL3NlbnRyeS9jb25mL3NlcnZlci5weQ==) | `89.17% <ø> (-0.02%)` | :arrow_down: | | [src/sentry/utils/canonical.py](https://app.codecov.io/gh/getsentry/sentry/pull/70026?src=pr&el=tree&filepath=src%2Fsentry%2Futils%2Fcanonical.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry#diff-c3JjL3NlbnRyeS91dGlscy9jYW5vbmljYWwucHk=) | `96.77% <100.00%> (-0.79%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/getsentry/sentry/pull/70026/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry)

[sentry-io[bot]]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ KeyError: 'start_timestamp' /api/0/organizations/{organization_slug}/events... View Issue

_{Did you find this useful? React with a 👍 or 👎}