Revert "Don't intercept accelerators in wxTextValidator in wxGTK."

Testing for absence of Alt modifier is wrong as it could be specified for the
events generated by AltGr-letter key combinations (AltGr is Alt+Ctrl), so this
commit broke the validation of any such symbols entered from non-US keyboard.

This is worse than the problem it was trying to fix, so revert this for now.
The real fix will need to ensure that the mnemonics are checked first, i.e.
before generating EVT_CHAR, in wxGTK, just as it already happens in wxMSW.

This reverts r75453.

See #15777.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75521 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775