docs: admin-guide: merge sections for the kernel.modprobe sysctl

docs: admin-guide: merge sections for the kernel.modprobe sysctl

Documentation for the kernel.modprobe sysctl was added both by
commit fd207f0fe38e ("docs: merge debugging-modules.txt into
sysctl/kernel.rst") and by commit 561f77febbd9 ("docs: admin-guide:
document the kernel.modprobe sysctl"), resulting in the same sysctl
being documented in two places.  Merge these into one place.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200414172430.230293-1-ebiggers@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>