kernel.bbclass: Remove do_install[prefunc] no longer needed

Prior work has refactored the do_install task multiple times, and any
references to PKGV and PKGR (even indirect ones) have been removed.

Signed-off-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 36fe4db8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>