Delete workaround for compat with extensions

[jakobnissen]

To allow the BioSequences FASTX extension to be supported with Julia <1.9, a few hacks had to be done.
Remove these hacks, as we now longer support these versions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.50%. Comparing base (e05e5ce) to head (8283273).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   89.78%   92.50%   +2.72%     
==========================================
  Files          15       15              
  Lines         695      694       -1     
==========================================
+ Hits          624      642      +18     
+ Misses         71       52      -19     

Flag	Coverage Δ
unittests	92.50% <ø> (+2.72%)	⬆️

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