Optimise IncrementalCacheManager.java

[ascopes]

Remove check for deleted files since the full rebuild check in 2.8.0 will already cover this eventuality, making the additional work redundant.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.56%. Comparing base (ef234bd) to head (3c5c56d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #504      +/-   ##
==========================================
+ Coverage   87.31%   87.56%   +0.26%     
==========================================
  Files          40       40              
  Lines        1315     1310       -5     
  Branches       91       89       -2     
==========================================
- Hits         1148     1147       -1     
+ Misses        124      122       -2     
+ Partials       43       41       -2     

Files with missing lines	Coverage Δ
...n/sources/incremental/IncrementalCacheManager.java	88.32% <100.00%> (+4.17%)	⬆️