CI: cirrus: drop CentOS 6 builds

CentOS 6 packages were removed from repo. Also, I was not able
to get it working using centos vault.

Further discussion on ML:

https://www.mail-archive.com/haproxy@formilux.org/msg38908.html

.cirrus.yml

@@ -12,20 +12,3 @@ FreeBSD_task:
     - ./haproxy -vv
     - ldd haproxy
     - env VTEST_PROGRAM=../vtest/vtest gmake reg-tests REGTESTS_TYPES=default,bug,devel || (for folder in /tmp/*regtest*/vtc.*; do cat $folder/INFO $folder/LOG; done && exit 1)
-
-centos_6_task:
-  container:
-    image: centos:centos6
-  only_if: $CIRRUS_BRANCH =~ 'master|next'
-  script:
-    - yum install -q -y gcc git openssl-devel pcre-devel epel-release socat
-    - yum install -q -y python34
-    - git clone https://github.com/VTest/VTest.git ../vtest
-    # Special flags due to: https://github.com/vtest/VTest/issues/12
-    - make -C ../vtest FLAGS="-O2 -s -Wall -lrt"
-    - make CC=cc V=1 TARGET=linux-glibc-legacy USE_ZLIB=1 USE_PCRE=1 USE_OPENSSL=1
-    - ./haproxy -vv
-    - ldd haproxy
-    # remove some reg-tests (CentOS 6 does not support alpn)
-    - rm reg-tests/{connection/proxy_protocol_random_fail,checks/tcp-check-ssl,connection/proxy_protocol_send_unique_id_alpn}.vtc
-    - env VTEST_PROGRAM=../vtest/vtest make reg-tests REGTESTS_TYPES=default,bug,devel || (for folder in /tmp/*regtest*/vtc.*; do cat $folder/INFO $folder/LOG; done && exit 1)