Remove RetryRule.

Demotion of flaky tests will be done at a method level instead
of module level (see b/292300420#comment4).

As such, it seems reasonable to remove the RetryRule completely
and instead fix these flakes one by one if they get demoted.

Bug: 286841597
Test: atest SafetyCenterFunctionalTestCases && atest SafetyCenterActivityFunctionalTestCases
Relnote: N/A
Change-Id: Ia418199c698c8230bf54615c49456fed56108eb1