linux: remove support of linux26-* targets

The linux-* mirror targets of linux26-* have been added a very long time ago
(2010) and linux 2.6 is now considered 'old' anyway. It no longer makes
sense to support these linux26-* targets, so this patch removes them.

This is a simplification introduced in preparation of the kconfig-package
infrastructure.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>