torture: Remove obsolete builddir options

The --builddir and --relbuilddir options were initially intended to handle
parallel tests.  However, since commit 43e38ab3

 (Enable concurrent
rcutorture runs), the script manages multiple build directories as
needed for parallel testing.  This commit therefore removes these two
obsolete options.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>