Build: rpm: drop no-longer-needed support tests from spec file

Drop tests we no longer need:

• We only support building on Fedora >=28, RHEL >=8, and SUSE >=15
• We require rpm 4.14, so we don't need to test for Recommends support
• All supported RPM distros use systemd
• The configure script no longer supports --enable-compat-2.0 or --enable-legacy-links
• concurrent-fencing now defaults to true
• We don't want a Conflicts with incompatible pcs versions, because some pcs commands will still work anyway, and it allows users to build a different Pacemaker without having to build pcs too if they don't need it