docs: drop incomplete build recipe for ubuntu 14.04

The kernel from such an old distro version likely has several unfixed
bugs.  Better not to support it at all.

Users who can upgrade the kernel are probably also sophisticated enough
to fix the build issues too.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

docs/install.md

@@ -73,9 +73,6 @@ within a temporary runtime directory.
 ### Ubuntu 18.10:
 `$ apt -y install build-essential btrfs-progs markdown && make`
 
-### Ubuntu 14.04:
-You can try to carry on the work done here: <https://gist.github.com/dagelf/99ee07f5638b346adb8c058ab3d57492>
-
 Packaging
 ---------