Remove manual mirroring in RTL case from wxGrid code.

This is not necessary as wxDC already inherits RTL from the window itself and,
in fact, breaks the display when using RTL.

Closes #16250.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77037 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775