Remove Combinatorics dependency

[TransGirlCodes]

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• ✨ New feature (A non-breaking change which adds functionality).
• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

📋 Additional detail

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.

[codecov]

Codecov Report

Merging #158 (1d2e0ca) into v3 (1bc7932) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##               v3     #158      +/-   ##
==========================================
- Coverage   80.16%   80.12%   -0.04%     
==========================================
  Files          31       31              
  Lines        2178     2179       +1     
==========================================
  Hits         1746     1746              
- Misses        432      433       +1     

Flag	Coverage Δ
unittests	80.12% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/BioSequences.jl	0.00% <ø> (ø)
src/longsequences/constructors.jl	77.41% <0.00%> (-1.27%)	⬇️

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