Remove amplitude views

[jklukas]

We never cleaned these up after removing the Amplitude export tasks in
mozilla/telemetry-airflow#1328

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If the PR comes from a fork, trigger integration CI tests by running the Push to upstream workflow and provide the <username>:<branch> of the fork as parameter. The parameter will also show up
  in the logs of the manual-trigger-required-for-fork CI task together with more detailed instructions.
• If adding a new field to a query, ensure that the schema and dependent downstream schemas have been updated

[jklukas]

We will need to manually delete the prod views once this is merged.

[jklukas]

Ran the following to clean up views:

bq rm moz-fx-data-shared-prod:firefox_accounts.fxa_amplitude_export
bq rm moz-fx-data-shared-prod:firefox_accounts.sync_send_tab_export
bq rm moz-fx-data-shared-prod:messaging_system.onboarding_events_amplitude
bq rm moz-fx-data-shared-prod:messaging_system.onboarding_retention_events_amplitude
bq rm moz-fx-data-shared-prod:telemetry_derived.devtools_events_amplitude_v1
bq rm mozdata:firefox_accounts.fxa_amplitude_export
bq rm mozdata:firefox_accounts.sync_send_tab_export
bq rm mozdata:messaging_system.onboarding_events_amplitude
bq rm mozdata:messaging_system.onboarding_retention_events_amplitude