Revert "Always define WXUSINGDLL when compiling Scintilla in shared wx build."

Revert "Always define WXUSINGDLL when compiling Scintilla in shared wx build."

Finally it's unnecessary to define WXUSINGDLL when building wxScintilla
library as it doesn't use the main DLL, it is simply used as part of it.

This reverts r66222 and finally closes #12626.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66238 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775