LU-12430 build: remove misleading help message

LU-12430 build: remove misleading help message

build/README.kernel-source has been deleted for nine years, but
the build help message still says: "Consult build/README.kernel-source
for details."

This patch removes these messages.

Test-Parameters: trivial
Change-Id: I2ff3afc363205b13f7f5ef645ade2854bfc3fa47
Signed-off-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/35205
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>