Don't use wxTE_PROCESS_ENTER in wxSpinCtrlGeneric.

Don't use wxTE_PROCESS_ENTER in wxSpinCtrlGeneric.

We don't need to catch Enter presses to generate spin control events as we do
it anyhow when we lose focus. So simply remove the Enter handling code
entirely (it could be used to generate wxEVT_COMMAND_TEXT_ENTER events but we
don't generate them nor wxEVT_COMMAND_TEXT_UPDATED at all currently in the
generic version).

Closes #12980.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67031 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775