Remove tzinfo dependency from Action Pack

This gem is used by Active Support but it should not be a dependency of
Action Pack.
Merge pull request #12262 from sferik/remove_tzinfo_from_actionpack

Remove tzinfo dependency from Action Pack
There's no need to do this

AS does the following inside Time.find_zone! ...
`ActiveSupport::TimeZone[time_zone] || TZInfo::Timezone.get(time_zone)`
and given that the test is stubbing AS::TZ[] we don't need the removed
code.