Add remove method to return Option<V> for unsync cache

[JokerXiL]

With current implementation of unsync cache, it is possible for the invalidate method to return previous cached value.

I modified it and put it as new remove method.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 91.60%. Comparing base (309fa89) to head (7cad7fd).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   91.84%   91.60%   -0.24%     
==========================================
  Files          23       23              
  Lines        4097     4112      +15     
==========================================
+ Hits         3763     3767       +4     
- Misses        334      345      +11     

[tatsuya6502]

Kani CI will never finish and will be canceled by GitHub Actions. Let's ignore it. We already have a workaround #41 for it on the v0.11.x branch.