[rubygems/rubygems] Remove unnecessary code

`@host` _could_ be `nil` at this point, but only if all the conditions
above for setting `@host` were `nil`. In that case, it is guaranteed
to `default_gem_server` metadata is `nil` since it's one of the
branches in that condition. So this code would just be setting an
already `nil` variable to `nil`. Hence, not needed.

rubygems/rubygems@e3ccff3c5c

lib/rubygems/commands/push_command.rb

@@ -72,10 +72,6 @@ def send_gem(name)
 
     gem_data = Gem::Package.new(name)
 
-    unless @host
-      @host = gem_data.spec.metadata['default_gem_server']
-    end
-
     push_host = nil
 
     if gem_data.spec.metadata.has_key?('allowed_push_host')