Revert "Fix flaky 3PID inhibit error unit tests"

[anoadragon453]

This PR reverts #19913, but leaves behind the comments in the tests noting that the endpoints have jitter.

See #19913 (comment) as for why. Kudos to @MadLittleMods for digging into the issue properly and providing an elegant fix in #19879!

---

928716e is the only new code here.

[MadLittleMods]

We could move this next to the relevant make_request inside _request_token(...)

[MadLittleMods]

Current comment works.

I could also see this explicit version being good:

Suggested change

	# Note: The endpoint intentionally adds up to 1000ms of jitter to avoid
	# leaking whether the email address is bound to an account.
	#
	# This should be handled by `await_result` underneath, which has a
	# 1000ms timeout.
	channel = self.make_request(
	"POST",
	b"register/email/requestToken",
	{"client_secret": "foobar", "email": email, "send_attempt": 1},
	# The endpoint intentionally adds up to 1000ms of jitter to avoid
	# leaking whether the email address is already bound to an account.
	timeout_ms=3000,
	)
	channel = self.make_request(
	"POST",
	b"register/email/requestToken",
	{"client_secret": "foobar", "email": email, "send_attempt": 1},
	await_result=False,
	)
	# Note: The endpoint intentionally adds up to 1000ms of jitter to avoid
	# leaking whether the email address is bound to an account.
	channel.await_result(timeout_ms=1000)