While here do some minor housekeeping;
* Remove superfluous `post-stage-MANPAGES-off` target, cleanup is already
handled with PLIST substitutions.
* Collapse the `PORT_OPTIONS` conditional pre-checks back into `DOCS`. We
only need sphinx when building the documentation, not `MANPAGES`, as such
disallow multiple jobs if `DOCS` is enabled.
Changes: https://github.com/fish-shell/fish-shell/releases/3.6.1
PR: 271558
Approved by: asomers (maintainer)
c35875d80d