Remove JWT_COOKIES_FLAG.

The JWT_COOKIES_FLAG was a temporary flag used for rollout of the new
JWT cookies. These are live in Production, so we are removing the flag.

Without this flag, we set JWT cookies during login. However, this
requires an oAuth Client that isn't always available during unit tests.
We introduced a feature flag that is only used for unit tests to
disable setting the JWT cookies. The code explains a bit more why this
solution was selected over adding the oauth client to the database.

ARCH-247