Remove *.pem substitutions in Dockerfile

[hannahilea]

In the Dockerfile, there's a section prefixed with a comment about

Adapted from bes/spec.all_static.in in RPM creation.
The four *.pem substitutions may be unnecessary, as those *.pem files may be
vestigial substitutions for a build process past.

Everyone on the team is pretty sure they are not necessary and can be removed; let's do so.

Note that we should absolutely do a -test-deploy with these changes on both hyrax-docker and SIT before formalizing the removal.