ci/libbuild.sh: drop yum/CentOS support

`dnf` is present in all the platforms we care about now, and the CentOS
bit is out of date. We can re-add it if we add e.g. C[89]S support with
the updated list of packages.

Motivated by noticing that the `yum` symlink isn't always present.

ci/libbuild.sh

@@ -6,7 +6,7 @@ OS_ID=$(. /etc/os-release; echo $ID)
 OS_VERSION_ID=$(. /etc/os-release; echo $VERSION_ID)
 
 pkg_upgrade() {
-    yum -y distro-sync
+    dnf -y distro-sync
 }
 
 make() {
@@ -20,7 +20,7 @@ build() {
 }
 
 pkg_install() {
-    yum -y install "$@"
+    dnf -y install "$@"
 }
 
 pkg_install_if_os() {
@@ -39,25 +39,15 @@ pkg_install_buildroot() {
             # https://github.com/projectatomic/rpm-ostree/pull/1889/commits/9ff611758bea22b0ad4892cc16182dd1f7f47e89
             # https://fedoraproject.org/wiki/Common_F30_bugs#Conflicts_between_fedora-release_packages_when_installing_package_groups
             if rpm -q fedora-release-container; then
-                yum -y swap fedora-release{-container,}
+                dnf -y swap fedora-release{-container,}
             fi
             pkg_install dnf-plugins-core @buildsys-build;;
-        centos) pkg_install yum-utils
-                # Base buildroot, copied from the mock config sadly
-                pkg_install bash bzip2 coreutils cpio diffutils system-release findutils gawk gcc gcc-c++ \
-                            grep gzip info make patch redhat-rpm-config rpm-build sed shadow-utils tar \
-                            unzip util-linux which xz;;
         *) fatal "pkg_install_buildroot(): Unhandled OS ${OS_ID}";;
     esac
 }
 
 pkg_builddep() {
-    # This is sadly the only case where it's a different command
-    if test -x /usr/bin/dnf; then
-        dnf builddep -y "$@"
-    else
-        yum-builddep -y "$@"
-    fi
+    dnf builddep -y "$@"
 }
 
 # Install both build and runtime dependencies for $pkg