centos: remove redundant code

Co-authored-by: Daijiro Fukuda <fukuda@clear-code.com>
Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

fluent-package/Gemfile

@@ -5,7 +5,7 @@ require File.expand_path('../config.rb', __FILE__)
 def platform_centos7?
   return false unless File.exist?("/etc/os-release")
   os_info = File.read("/etc/os-release")
-  os_info.include?("CentOS Linux 7") ? true : false
+  os_info.include?("CentOS Linux 7")
 end
 
 source "https://rubygems.org"