Clean up Partitioned Epoch Rewards feature logic

[CriesofCarrots]

Problem

The feature gate partitioned_epoch_rewards_superfeature has been activated on all public clusters, but the gating logic remains.

Summary of Changes

• Remove feature gate logic, primarily in bank.rs, but also the superfeature fix in bank::partitioned_epoch_rewards::sysvar::create_epoch_rewards_sysvar()
• Fixup some tests to expect PER
• Remove pre-PER epoch-boundary rewards code, now unused
• Cherry-pick @HaoranYi 's commit to remove partitioned reward test code

Fixes #426

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[brooksprumo]

This file already has a top-level #![cfg(test)], is this one also needed?

Suggested change

#[cfg(test)]

[brooksprumo]

Is the actual logic changing in these tests? If yes, wdyt about making those changes in a separate PR? Or are the changes coupled with removing the featurization logic?

[HaoranYi]

A question out of the scope of this PR - Are those stats useful to us? Should we port them to partitioned reward code path?

[HaoranYi]

Nice cleanup. One of my favorite type of PRs - deleting code!
I have just one question to consider, probably to address in a follow up PR.

[pgarg66]

Approving from SVM