The recipe was previously given in comments in the module's test
script, but now we have an explicit recipe in the Makefile. The now
redundant comments in the script are removed.

This recipe shouldn't be needed in normal use, as the certificate and
key are in git and don't need to be regenerated.

Discussion: https://postgr.es/m/ae8f21fc-95cb-c98a-f241-1936133f466f@2ndQuadrant.com