Drop custom libuv from CentOS 6

CentOS 6 image is used only for BIND 9.11 which does not use libuv.
4 jobs for !107 with 12-drop-custom-libuv-from-centos6 in 10 minutes and 39 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Images
passed #1555781
amd64 linux docker
centos:6:amd64:docker

00:08:55

passed #1555780
i386 linux docker
centos:6:i386:docker

00:10:39

passed #1555782
amd64 linux docker
centos:7:amd64:docker

00:05:46

passed #1555783
amd64 linux docker
centos:8:amd64:docker

00:04:51