remove duplicate unique-prx constraint from pull requests table

[MoralCode]

augur/augur/application/db/models/augur_data.py

Lines 1711 to 1716 in 2b3f8de

	class PullRequest(Base):
	__tablename__ = "pull_requests"
	__table_args__ = (
	UniqueConstraint("repo_id", "pr_src_id", name="unique-pr"),
	UniqueConstraint("repo_id", "pr_src_id", name="unique-prx"),
	UniqueConstraint("pr_url", name="pull-request-insert-unique"),

the unique-prx constraint is entirely redundant and can be removed.

Originally posted by @MoralCode in #3192

[cdolfi]

UniqueConstraint("pr_url", name="pull-request-insert-unique") also needs to be removed as a unique constraint. The unique constraint needed is:
UniqueConstraint("pr_src_id", name="unique-pr") (or whatever naming connotation you'd like)

pr_url can change if the repo is moved. So if the name of the repo changes this unique constraint will not identify those PRs. The one thing that stays consistent is the pr_src_id. Changing to be based on the src_id is what has fixed this problem in other parts of the codebase

[MoralCode]

This is a separate issue from the one identified in #3192