ash: remove superfluous code in arithmetic mode

ash: remove superfluous code in arithmetic mode

Based on commits 1a74845, cfc3d6a and ff13779 from
git://git.kernel.org/pub/scm/utils/dash/dash.git by Herbert Xu

function                                             old     new   delta
evalcommand                                         1197    1204      +7
localcmd                                             327     325      -2
readtoken1                                          3200    3180     -20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 7/-22)             Total: -15 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>