Cleanup: remove the read_elf_kcore()

Here, no need to wrap the read_elf() again, lets invoke it directly.
So remove the read_elf_kcore() and clean up redundant code.

Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>