(maint) drop unneeded clear_pool helper

Given the way we're flushing redis (which seems super performant), we don't
need to clear pools any more at the beginning of tests.

spec/helpers.rb

@@ -39,9 +39,3 @@ end
 def fetch_vm(vm)
   redis.hgetall("vmpooler__vm__#{vm}")
 end
-
-def clear_pool(pool)
-  ['ready'].each do |state| # TODO add more states if desired
-    redis.del("vmpooler__#{state}__#{pool}")
-  end
-end