chore/dotcom: delete subscriptions, licensing, and cody gateway usage

[bobheadxi]

With the launch of Enterprise Portal and migration of various dependents, we can now delete:

• all GraphQL mutations and queries for subscriptions, licensing, and cody gateway usage
• all DB handlers for subscriptions and licensing
• all background workers for licensing

Closes https://linear.app/sourcegraph/issue/CORE-249

Test plan

[bobheadxi]

[!WARNING] This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite. Learn more

• #64506 👈
• #64479
• #64424 : 1 other dependent PR (#64504 )
• #64400
• #64396
• #64253 : 2 other dependent PRs (#64057 , #64115 )
• #63959
• #64114
• #63927
• #64087
• #64317
• #64090
• #64376
• main

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