Results collection interrupted due to disk capacity limits

On 2018-12-01 and 2018-12-02, we failed to collect test results from either release of Safari. This was because the Buildbot master had reached hard disk capacity. Much of the storage was used by old Linux kernel headers which were no longer needed.

We resolved this for the GNU/Linux-powered Buildbot worker machines in July via gh-581. As an operating system concern, it is also relevant for the GNU/Linux Buildbot master machines. That it was not applied to the Buildbot masters was an error of omission.

web-platform-tests / results-collection

Results collection interrupted due to disk capacity limits #637