Remove unused field FixtureManager._arg2finish
Remove unused field FixtureManager._arg2finish

Not used since 4f0879ff9b3e7ad0da1839131c35b4e6412b972f.

pytest-dev/pytest,alfredodeza/pytest,markshao/pytest,nicoddemus/pytest,Akasurde/pytest,The-Compiler/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest