Don't manually centre dialogs created with default position in wxMSW.

Don't manually centre dialogs created with default position in wxMSW.

We always centered the dialogs on the main display which was wrong if the
parent window was on another one. Instead of fixing it, simply don't centre
them at all and let Windows position them, there is no reason to change the
default behaviour.

Closes #13387.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68606 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775