Removing “dependency-check” target from build.xml

[lewis john mcgibbney <le...@apache.org>]

Hi dev@,

Recently I was doing a bit of work on CI and made an attempt to activate
the “dependency-check” target (previously named “report-vulnerabilities”).

It appears that the underlying “dependency-check” tooling is flaky at best.
It appears to take an awful long time to execute and seems to be prone to
hanging.

I propose to remove this target and implement something more stable in the
future… when I work on finishing the Gradle build.

lewismc