r/haproxy: Clean up for modern haproxy versions

The current version of *haproxy* packaged in Fedora already enables
configuration via fragments in a drop-in directory, though it uses
a different path by default.  I still like separating the global
configuration from the defaults, though, and keeping the main
`haproxy.cfg` file empty.

roles/haproxy/files/haproxy-config.conf

@@ -1,2 +0,0 @@
-[Service]
-Environment=CONFIG=/etc/haproxy