Remove rubygem(thin) dependency for good.

It seems that Thin development has slowed down as well as its
popularity. Due to this, the Thin handler was removed from Rack [1] and
moved back to Thin [2].

Based on this, removing the dependency will help us:

1) Remove the circular dependecy once and forever.
2) Eventualy remove rubygem-thin from Fedora.

Not testing Rack against Thin won't be a big loss.

[1]: 98d9cf5834
[2]: b201525f0d

rubygem-rack.spec

@@ -1,8 +1,5 @@
 %global gem_name rack
 
-# There is circular dependency between thin and rack.
-%bcond_with bootstrap
-
 Name: rubygem-%{gem_name}
 Version: 2.2.3
 # Introduce Epoch (related to bug 552972)
@@ -23,9 +20,6 @@ BuildRequires: rubygem(concurrent-ruby)
 BuildRequires: memcached
 BuildRequires: rubygem(memcache-client)
 BuildRequires: rubygem(minitest)
-%if ! %{with bootstrap}
-BuildRequires: rubygem(thin)
-%endif
 BuildArch: noarch
 
 %global __brp_mangle_shebangs_exclude_from ^%{gem_instdir}/test/cgi/test.ru$
@@ -128,6 +122,7 @@ popd
 %changelog
 * Wed Oct 21 2020 Vít Ondruch <vondruch@redhat.com> - 1:2.2.3-3
 - Re-enable test suite.
+- Remove rubygem(thin) dependency for good.
 
 * Wed Jul 29 2020 Fedora Release Engineering <releng@fedoraproject.org> - 1:2.2.3-2
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_33_Mass_Rebuild