Remove abi-tool directory

This commit removes vndk/tools/abi-tool directory.  It is an attempt to
leverage existing abi-dumper and abi-compliance-checker.  However, this
plan does not work well.  We are developing an alternative in the
header-checker directory.

Test: none
Change-Id: I19b859fef5b980168b3f500659fa39a8c52f09eb