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>