Throwing out `TestLinks`

It was just retesting the link-related command tests for some reason...?
Fixing `splice` call to be ECMAScript compliant

Turns out that Chrome and Firefox (and probably others) understand that
a one-arg `splice` call is intended to drop one element, but brainiac
Nashorn interprets it as "ignore this function call because it doesn't
have all of the arguments that I want" and silently goes about its
business.

In all fairness to Nashorn, the ECMAScript version of the function
_does_ require two arguments.  In all fairness to me, though, Mr.
Nashorn, FFFFFFFUUUUUUUUUUUUU!
Ensuring that the `LinkManager` gets cleared when appropriate
Having turtles only poll their own links

Amusingly, turtles were tracking their own links, but doing pretty much
nothing with that value.

On the more serious side of things, this change should offer a
substantial performance benefit for models that use links, and it was
really a necessary change for testability, since iterating over
`world.links()` would ultimately mess with RNG state and the iteration
order of the resultant agentsets.