Fix contrib/balance_pool to work with bash 4.2.25.

Bash version 4.3.11 supports this construct:
    unset FUBAR[-1]
... meaning, remove the last element from array FUBAR.

Alas, cautotest is running bash 4.2.25, which doesn't support the
construct.  We want to run this script from cautotest, so change to
an equivalent construct that *is* supported.

BUG=None
TEST=try it out on my desktop (with 4.3.11, <sigh>)

Change-Id: Ic654922d831e0282c463548ad66be289db77e835
Reviewed-on: https://chromium-review.googlesource.com/226810
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Mungyung Ryu <mkryu@google.com>
1 file changed