Remove unused test code

[CriesofCarrots]

Problem

#31007 removed overrides from mock_process_instruction(), but a stake-redelegate test still has a sysvar cache override block. This is because it is mutable and mutated, so doesn't alert the compiler that it is unused.
The lingering block is confusing to people reading this test to understand how to handle sysvars in stake tests.

Summary of Changes

Remove unused block

[codecov]

Codecov Report

Merging #33247 (92c61fd) into master (602f0f6) will decrease coverage by 0.1%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #33247     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
=========================================
  Files         790      790             
  Lines      214214   214208      -6     
=========================================
- Hits       175585   175562     -23     
- Misses      38629    38646     +17