Don't delegate to users `sbt`.

This changes our `sbt/sbt` script to not delegate to the user's `sbt`
even if it is present. If users already have sbt installed and they
want to use their own sbt, we'd expect them to just call sbt directly
from within Spark. We no longer set any enironment variables or anything
from this script, so they should just launch sbt directly on their own.

There are a number of hard-to-debug issues which can come from the
current appraoch. One is if the user is unaware of an existing sbt
installation and now without explanation their build breaks because
they haven't configured options correctly (such as permgen size)
within their sbt. Another is if the user has a much older version
of sbt hanging around, in which case some of the older versions
don't acutally work well when newer verisons of sbt are specified
in the build file (reported by @marmbrus). A third is if the user
has done some other modification to their sbt script, such as
setting it to delegate to sbt/sbt in Spark, and this causes
that to break (also reported by @marmbrus).

So to keep things simple let's just avoid this path and
remove it. Any user who already has sbt and wants to build
spark with it should be able to understand easily how to do it.