Remove old migration code

[jasonforal]

What problem are you trying to solve?

https://github.com/DataDog/datadog-static-analyzer/pull/446 preserved the --ddsa-runtime CLI flag to provide a graceful migration. Removing it is long overdue.

What is your solution?

Delete the old code.

Alternatives considered

What the reviewer should know

• All internal usages of this flag have been removed.