Cleanup codebase.

[MohitMaliFtechiz]

Fixes #4490

• Removed unused code, classes, and extension functions that are no longer in use.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.12%. Comparing base (709c181) to head (ccbe85d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rg/kiwix/kiwixmobile/core/utils/files/FileUtils.kt	0.00%	7 Missing ⚠️
...xmobile/zimManager/FileWritingFileSystemChecker.kt	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4491      +/-   ##
============================================
+ Coverage     58.67%   59.12%   +0.45%     
+ Complexity     1470     1461       -9     
============================================
  Files           327      320       -7     
  Lines         17066    16908     -158     
  Branches       2116     2095      -21     
============================================
- Hits          10013     9997      -16     
+ Misses         5648     5508     -140     
+ Partials       1405     1403       -2     

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