UBUNTU: snapcraft.yaml: stop invoking the obsolete (and non-existing) 'firmware_insta...

UBUNTU: snapcraft.yaml: stop invoking the obsolete (and non-existing) 'firmware_install' target

BugLink: http://bugs.launchpad.net/bugs/1782166
The snapcraft.yaml in bionic/master (by default) invokes the now obsolete
'firmware_install' target that was removed in
5620a0d1aacd554ebebcff373e31107bb1ef7769 ("firmware: delete in-kernel
firmware"), and by invoking a non existing target, it fails to build.

Since firmware files were removed from the Linux source code, and we rely on
external files provided by the linux-firmware package, skip invoking
'firmware_install' altogether and fix the build.

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>