Replace legacy is_function_available() with fact

The `is_pe` fact is present on modern PE installations.
is_function_available() got removed in stdlib 9:
puppetlabs/puppetlabs-stdlib@0278618

manifests/params.pp

@@ -40,12 +40,7 @@
   $ssldir = "${confdir}/ssl"
   $config = "${confdir}/puppet.conf"
 
-  # The `is_pe` fact currently works by echoing out the puppet version
-  # and greping for "puppet enterprise". With Puppet 4 and PE 2015.2, there
-  # is no longer a "PE Puppet", and so that fact will no longer work.
-  # Instead check for the `is_pe` fact or if a PE provided function is available
-  $_is_pe = (getvar('::is_pe') or is_function_available('pe_compiling_server_version'))
-  if $_is_pe {
+  if $facts['is_pe'] {
     # Calculate the default collection
     $_pe_version = pe_build_version()
     # Not PE or pe_version < 2018.1.3, use PC1