Remove deprecated Concurrent.disable_at_exit_handlers! from acceptance test helper.rb since it now has no effect, see ruby-concurrency/concurrent-ruby#841

Author: quartzmo

google-cloud-spanner/acceptance/spanner_helper.rb

@@ -31,9 +31,6 @@ def SecureRandom.int64
   random_bytes(8).unpack("q")[0]
 end
 
-# Disable exit handlers because it messes with minitest/autorun
-Concurrent.disable_at_exit_handlers!
-
 # Create shared spanner object so we don't create new for each test
 $spanner = Google::Cloud::Spanner.new