tools/libelf: remove unneeded host library

tools/libelf: remove unneeded host library

This old ELF library dating to 2009 used to be necessary on MacOS but
is not required for building the kernel or tools since [1]. On Linux
systems, libelf is already an OpenWRT build-system prerequisite [2].

Presence of the older library can mask or conflict with the system libelf
and lead to build errors, as seen compiling Linux kernels since v5.8 or
host tools such as dwarves (e.g. pahole).

Remove the unnecessary tools/libelf library and avoid the related issues.

[1] 5f8e587240 ("build: force disable stack validation during kernel build
                 on non-linux systems")
[2] https://openwrt.org/docs/guide-developer/build-system/install-buildsystem#prerequisites

Tested-by: Rosen Penev <rosenp@gmail.com> (Linux)
Tested-by: Georgi Valkov <gvalkov@abv.bg> (MacOS)
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>