Removes AppendVec::file_name()

[brooksprumo]

Problem

There's already a function for getting the file name for a storage: AccountStorageEntry::file_name(). And AccountStorageEntry is also what holds the slot and id. AppendVec doesn't know about this info, so use AccountsFile instead.

Summary of Changes

Replace calls to AppendVec::file_name() with AccountsFile::file_name().

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (14d6c79) to head (8c99a57).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #976     +/-   ##
=========================================
- Coverage    81.9%    81.8%   -0.1%     
=========================================
  Files         853      853             
  Lines      231797   231799      +2     
=========================================
- Hits       189845   189830     -15     
- Misses      41952    41969     +17     

[HaoranYi]

lgtm