refactor: move benchmark to its own repo

It's now at https://github.com/coveragepy/benchmark

Author: nedbat

MANIFEST.in

@@ -30,8 +30,6 @@ include tox.ini
 
 recursive-include ci *
 recursive-include lab *
-recursive-include benchmark *
-exclude benchmark/results.json
 recursive-include .github *
 
 recursive-include coverage *.pyi