(maint) Unpin async gem

Previously, the async gem needed to be pinned to 1.30 because of the
version of Ruby that internal Jenkins runners were using. We are now
using more modern Rubies, so we do not need to pin the gem any longer.

Author: mhashizume

.sync.yml

@@ -40,8 +40,6 @@ Gemfile:
       - gem: 'orchestrator_client'
         version: '< 0.7.1'
         condition: 'ENV["GEM_BOLT"]'
-      - gem: async
-        version: '~> 1.30' # otherwise async 2.0.0(needs ruby >=3.1.0) is wrongly selected by bundler on jenkins while running with ruby 2.7.1
     ":system_tests":
       - gem: voxpupuli-acceptance
         version: '~> 3'