ref(flags): Remove organizations:performance-mep-reintroduce-histograms

[wedamija]

Dev-only flag registered Oct 2022, never rolled out to customers. Remove the flag registration and the unreleased gated code paths.

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.57%

[github-actions]

Backend Test Failures

Failures on b7fdec9 in this run:

tests/symbolicator/test_minidump_full.py::SymbolicatorMinidumpIntegrationTest::test_minidump_threadnames — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/symbolicator/test_minidump_full.py:207: in test_minidump_threadnames
    event = self.post_and_retrieve_minidump({"upload_file_minidump": f}, {})
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/sentry/testutils/relay.py:127: in post_and_retrieve_minidump
    assert resp.ok
E   assert False
E    +  where False = <Response [503]>.ok