net/activemq: Update to 5.15.12

Clean up the port a little and drop build time configuration knobs
MQDB, MQUSER, MQGROUP. All of these have runtime equivalents that
can be set in rc.conf instead.

Changes: https://issues.apache.org/jira/secure/ReleaseNote.jspa?version=12346500&styleName=&projectId=12311210