(maint) remove reference to ext/puppet-load.rb

In commit a8be1158d, the file `ext/puppet-load.rb` was deleted.
However, its reference for our packaging systems in
`ext/redhat/puppet.spec.rb`, causing our packaging on EL to fail.

This commit removes the reference in ext/redhat/puppet.spec.erb.
Merge pull request #3288 from rji/maint/master/el-packaging-fixup

(maint) remove reference to ext/puppet-load.rb