devtools: remove old ABI validation script

Since we've moved away from our initial validate-abi.sh script,
in favor of check-abi.sh, which uses libabigail,
remove the old script from the tree, and update the docs accordingly.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>