mandoc: Don't brand Volume titles

[me_cameronkatri.com]

Summary

msec.in is provided to add custom volumes, like 3lua, or to use a
different convention, not to brand. The operation system is shown in the
footer anyways.

[me_cameronkatri.com]

mandoc 1.14.6 was just released on September 23 (https://mandoc.bsd.lv/NEWS), but Phabricator isn't really suited to submit patches for vendor updates, so a committer will have to do that.

[me_cameronkatri.com]

Rationale: https://marc.info/?l=mandoc-discuss&m=163333478800796