Move RewardInterval to runtime

[LucasSte]

Problem

enum RewardInterval is only used in runtime, but it is declared in accounts-db.

Summary of Changes

I moved the enum to runtime/src/bank.rs, closer to its usage.

[codecov]

Codecov Report

Merging #34146 (d27f5d2) into master (4cbd57e) will increase coverage by 0.0%.
Report is 4 commits behind head on master.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34146   +/-   ##
=======================================
  Coverage    81.9%    81.9%           
=======================================
  Files         818      818           
  Lines      219945   219945           
=======================================
+ Hits       180199   180204    +5     
+ Misses      39746    39741    -5