Revert "Correctly set toolchain prefix for GCC cross compilers."

This reverts commit 8ee37bfaed662dcae521c4c901ea91aec3258547.
For MinGW this code is irrelevant and for gcc it never had any effect as
the property values are overwritten right below the call to
setCommonProperties.

Change-Id: I6784d992fd4488ef479425a98a075c48c74549d7
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>