Brave Origin builds compile out Brave Rewards
(enable_brave_rewards = !is_brave_origin_branded), but the
BraveRewardsDisabled policy was still registered unconditionally, so
brave://policy listed it on Brave Origin builds even though the feature
isn't present.
Wrap the BraveRewardsDisabled entry in kBraveSimplePolicyMap and the
matching kBraveOriginProfileMetadata entry in #if
BUILDFLAG(ENABLE_BRAVE_REWARDS), mirroring how Wallet, Talk, AI Chat,
etc. are gated. The unit test assertions and BUILD.gn rewards deps move
under the same buildflag.
Fixesbrave/brave-browser#55558