regexp: revert "use sync.Pool to cache regexp.machine objects"
Revert CL 101715.
The size of a sync.Pool scales linearly with GOMAXPROCS,
making it inappropriate to put a sync.Pool in any individually
allocated object, as the sync.Pool documentation explains.
The change also broke DeepEqual on regexps.
I have a cleaner way to do this with global sync.Pools but it's
too late in the cycle. Will revisit in Go 1.12. For now, revert.