remove centos6 build, CentOS has reached EOL

Author: jspaleta

.bonsai.yml

@@ -18,15 +18,6 @@ builds:
   -  "entity.system.os == 'linux'"
   -  "entity.system.arch == 'amd64'"
   -  "entity.system.platform == 'alpine'"
-- platform: "centos6"
-  arch: "amd64"
-  asset_filename: "#{repo}_#{version}_centos6_linux_amd64.tar.gz"
-  sha_filename: "#{repo}_#{version}_sha512-checksums.txt"
-  filter:
-  -  "entity.system.os == 'linux'"
-  -  "entity.system.arch == 'amd64'"
-  -  "entity.system.platform_family == 'rhel'"
-  -  "entity.system.platform_version.split('.')[0] == '6'"
 - platform: "centos7"
   arch: "amd64"
   asset_filename: "#{repo}_#{version}_centos7_linux_amd64.tar.gz"