Remove Drush's cache API. We no longer need to cache annotation parsing.

[weitzman]

No description provided.

[weitzman]

This is green now, and the benchmarks look good. In fact, we are now faster without the cache for some reason. Its a mystery but I'm not gonna sweat it.

Branch	Mean [ms]	Min [ms]	Max [ms]	Relative
11.x	99.3 ± 4.2	92.2	112.6	1.05 ± 0.05
this PR	94.7 ± 2.1	91.7	99.8	1.00

This results from running hyperfine.

hyperfine --prepare "git checkout 11.x"  "./drush" \  
          --prepare "git checkout remove-cache-api" "./drush" --export-markdown results.md