remove qtPrepareTool()'s ability to run batch files

it was only meant to automatically support syncqt.bat, which is gone
now.

fwiw, invoking batch files from within msys Makefiles was broken to
start with, as sh cannot directly run them.

Change-Id: I435568c578ce79e46f4e230e985ca9a04b34ffff
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>