Remove platforms with useless compilers gcc 7 and gcc 8

.gitlab-ci.yml

conan-el6-gcc73-x86/.gitlab-ci.yml

conan-el6-gcc73-x86/Dockerfile

conan-el6-gcc73-x86/docker-compose.yml

conan-el6-gcc73/.gitlab-ci.yml

conan-el6-gcc73/Dockerfile

conan-el6-gcc73/docker-compose.yml

conan-el6-gcc83-x86/.gitlab-ci.yml

conan-el6-gcc83-x86/Dockerfile

conan-el6-gcc83-x86/docker-compose.yml

conan-el6-gcc83/.gitlab-ci.yml

conan-el6-gcc83/Dockerfile

conan-el6-gcc83/docker-compose.yml

conan-el7-gcc73-x86/.gitlab-ci.yml

conan-el7-gcc73-x86/Dockerfile

conan-el7-gcc73-x86/docker-compose.yml

conan-el7-gcc73/.gitlab-ci.yml

conan-el7-gcc73/Dockerfile

conan-el7-gcc73/docker-compose.yml

conan-el7-gcc82-x86/.gitlab-ci.yml