Remove LocalCache in FileStore

[casperisfine]

Ref: #42611

Context

LocalCache used to keep the deserialized value in memory, however this opened the door for the cached value to be mutated (see #36656 and #37587). So it used to both save the backend rountrip, and the deserialization cost.

But now that we almost always need to deserialize local cache entries, LocalCache now at best only save the backend rountrip. And in the case of the FileStore it's pretty much nothing.

Benchmark

I used a slightly modified version of @hoangtuyb96's benchmark: https://gist.github.com/casperisfine/80620e26aaf786a9d3016d5823ade15b

$ DISABLE_LOCAL_CACHE=1 SIZE=10 ruby bench-as-cache.rb 
fetch in rails 7.0.0.alpha
                        258.287  (± 1.5%) i/s -      1.300k in   5.034323s
write in rails 7.0.0.alpha
                          1.776k (± 3.3%) i/s -      8.960k in   5.050029s
$ SIZE=10 ruby bench-as-cache.rb 
fetch in rails 7.0.0.alpha
                        233.276  (± 6.9%) i/s -      1.175k in   5.065071s
write in rails 7.0.0.alpha
                          1.476k (±10.3%) i/s -      7.238k in   5.030478s

$ DISABLE_LOCAL_CACHE=1 SIZE=1000 ruby bench-as-cache.rb 
fetch in rails 7.0.0.alpha
                        228.549  (± 6.6%) i/s -      1.144k in   5.026979s
write in rails 7.0.0.alpha
                        129.034  (± 2.3%) i/s -    648.000  in   5.024184s

$ SIZE=1000 ruby bench-as-cache.rb
fetch in rails 7.0.0.alpha
                        234.919  (± 7.2%) i/s -      1.170k in   5.011433s
write in rails 7.0.0.alpha
                         67.769  (± 1.5%) i/s -    342.000  in   5.048530s

So LocalCache is pretty much useless for FileStore.