Don't delete file when file is already being deleted

[renefloor]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bugfix

⤵️ What is the current behavior?

When a file is old and over capacity the store tries to delete the file twice

🆕 What is the new behavior (if this is a feature change)?

It only deletes it once.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

I wrote a test, but it didn't fail, because file.exists() is very fast with memory filesystem. Automated testing only realy works when you force a delay in the code, which is not nice.

📝 Links to relevant issues/docs

fixes #184 and fixes Baseflow/flutter_cached_network_image#335

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[codecov]

Codecov Report

Merging #185 into develop will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff            @@
##           develop     #185   +/-   ##
========================================
  Coverage    73.28%   73.28%           
========================================
  Files           13       13           
  Lines          423      423           
========================================
  Hits           310      310           
  Misses         113      113           

Impacted Files	Coverage Δ
flutter_cache_manager/lib/src/cache_store.dart	87.91% <83.33%> (ø)

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