Retire buildbot documentation

The CI has been fully converted to use GitHub Actions.  Retire
the now dead links to buildbot and associated documentation.

Signed-off-by: Brian Behlendorf <[email protected]>

Author: behlendorf

docs/Developer Resources/Buildbot Options.rst

@@ -8,9 +8,6 @@ Developer Resources
 
    Custom Packages
    Building ZFS
-   Buildbot Status <https://build.openzfs.org/tgrid?length=100&branch=master&category=Platforms&rev_order=desc>
-   Buildbot Known Issues <https://build.openzfs.org/known-issues.html>
-   Buildbot Options
    OpenZFS Patches
    OpenZFS Documentation <https://openzfs.org/wiki/Developer_resources>
    Git and GitHub for beginners