Only allow BankForks creation with single bank

[steviez]

Problem

BankForks has two constructors; one that takes a single Bank (the root) and one that can take an arbitrary number of Banks plus the root slot. However, the constructor that accepts multiple banks is unnecessary; it isn't used in production and is only used in several tests.

Summary of Changes

• Remove the BankForks constructor that allows multiple bank
• Update unit tests that used the multi-bank constructor

[codecov]

Codecov Report

Merging #34449 (9cac54c) into master (2dfcdce) will decrease coverage by 0.1%.
Report is 5 commits behind head on master.
The diff coverage is 97.8%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #34449     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         819      819             
  Lines      220923   220907     -16     
=========================================
- Hits       180870   180844     -26     
- Misses      40053    40063     +10     

[apfitzge]

lgtm