blockstore: Remove unnecessary function and threadpool

[steviez]

Problem

This is somewhat of a follow on to solana-labs#34768. Namely, that PR removed a threadpool that was used to fetch entries. There is a similar thread pool that is used to fetch entries for the use-case of purging special columns. Regardless of the use-case, the sentiment that we shouldn't be using a threadpool to fetch the entries remains

Summary of Changes

• Update Blockstore::purge_special_columns_exact() to use the public facing function
  • Use of allow_dead_slots == true retains previous functionality
• Remove the now unused function
• Removed the now unused threadpool

Note that the thread-pool was inside a lazy_static!, and would have only been accessed when Blockstore::clear_unconfirmed_slot() was called. That function gets called from ReplayStage when something is wrong (like the node forking off). So, any healthy node should not created this pool. Regardless, it is still good to cleanup

Work is part of #35

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (b6f6fdb) to head (fb73572).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #122     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         838      838             
  Lines      225947   225933     -14     
=========================================
- Hits       184923   184894     -29     
- Misses      41024    41039     +15     

[AshwinSekar]

lgtm 🪓