Don't center wxGenericMessageDialog elements.

Don't center wxGenericMessageDialog elements.

Centering doesn't seem to be appropriate on neither of the major platforms and
makes wxGenericMessageDialog and wxGenericRichMessageDialog which uses it look
even less native than otherwise.

Simply don't do it neither for the text nor, especially, for the buttons where
wxStdDialogButtonSizer already exists to take care of their alignment.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65450 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775