Revert "kernel.bbclass: enable a link for external module building"

This reverts commit 3224472c68865714716305dfbc8efaa59f73958e:

  Author: Nitin A Kamble <nitin.a.kamble@intel.com>
  Date:   Tue Sep 30 14:40:05 2014 -0700

    kernel.bbclass: enable a link for external module building

    Even though the kernel-dev package provides the required support for
    building external kernel modules on the target, some commonly used scripts
    and utilities fail as they are not finding the kernel module build support
    files at the desired location.

    Create the /lib/modules/<kernel-version>/build link on target pointing to
    the sources provided by the kernel-dev package, to fix the issue.

    Fixes Bug:
    [YOCTO #2968]

This patch completely ignores kernels that disable modules, which
micro-galileo does.