Removes unused parameter from AccountsDb::new_existing()

[brooksprumo]

Problem

AccountsDb::new_existing() has an unused parameter, num_accounts, that can be removed.

Summary of Changes

Remove it.

[mergify]

automerge label removed due to a CI failure

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.8%. Comparing base (2ac2f8e) to head (adc07d8).
⚠️ Report is 3170 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6543       +/-   ##
===========================================
+ Coverage        0    82.8%    +82.8%     
===========================================
  Files           0      849      +849     
  Lines           0   379196   +379196     
===========================================
+ Hits            0   314132   +314132     
- Misses          0    65064    +65064     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.