Simplify narayana.sh by removing ability to have empty IPV6_OPTS

We currently use [ -z "${VAR+c}" ] notation to detect if a variable has been set, allowing for it being set to the empty string. Here's an example:

  # if IPV6_OPTS is not set get the jdbc drivers (we do not run the jdbc tests in IPv6 mode)
  [ -z "${IPV6_OPTS+x}" ] && ant -DisIdlj=$IDLJ "$QA_BUILD_ARGS" get.drivers dist ||
    ant -DisIdlj=$IDLJ "$QA_BUILD_ARGS" dist

The problem with this approach is that it's rather cryptic to understand what's going on. This makes it hard to read and easy to create a bug. We've had two bugs so far, due to this syntax.

By removing the ability to "export IPV6_OPTS=", we can simplify these portions of the script to look like:

if [ "${IPV6_OPTS}" = "" ]; then
	ant -DisIdlj=$IDLJ "$QA_BUILD_ARGS" get.drivers dist
else
	ant -DisIdlj=$IDLJ "$QA_BUILD_ARGS" dist
fi

Which is a lot easier to read and much less error-prone.

Should we ever need to enable IPv6 without setting any IPv6 options, we could either re-instate the old code, or set a dummy option.