Removes Default and pub from CumulativeOffset

[brooksprumo]

Problem

1. CumulativeOffset and its fields are marked public, but are only used within accounts_hash.rs.
2. With Optimize account hash CumulativeOffset index from vec to 2-element array #33839, the type of index will change to a two-element array. A default doesn't necessarily make sense, as we'll always need at least one valid index to be set.

Summary of Changes

Remove pub and Default.

[brooksprumo]

Since this trait was public, and the extract function takes a CumulativeOffset, that meant CumulativeOffset needed to be public. But extract is only called in this file, so the trait does not need to be public either.

[HaoranYi]

lgtm

[codecov]

Codecov Report

Merging #33840 (b3dd36d) into master (6470544) will increase coverage by 0.0%.
Report is 2 commits behind head on master.
The diff coverage is 100.0%.

❗ Current head b3dd36d differs from pull request most recent head 00ce0c2. Consider uploading reports for the commit 00ce0c2 to get more accurate results

@@           Coverage Diff           @@
##           master   #33840   +/-   ##
=======================================
  Coverage    81.8%    81.9%           
=======================================
  Files         809      809           
  Lines      217712   217627   -85     
=======================================
- Hits       178295   178250   -45     
+ Misses      39417    39377   -40     

[brooksprumo]

Rebased and force-pushed to address merge conflict. No actual changes were made.