Remove dash.preinst

The preinst had been necessary to unpack dash when /bin/sh was also
included in the bash package, but bash stopped shipping /bin/sh over
six years ago.  Nowadays the preinst seems entirely superfluous and
prevents crossgrades, as well as spitting out noise on upgrades.

Closes: #907132, #890073

Signed-off-by: Sven Joachim <svenjoac@gmx.de>