Remove wxDECLARE_APP() from wxIMPLEMENT_APP_NO_MAIN().

Remove wxDECLARE_APP() from wxIMPLEMENT_APP_NO_MAIN().

There is no need for a separate declaration inside wxIMPLEMENT_APP() as
wxGetApp() is defined by it and redeclaring it immediately after definition
results in a warning with some compilers/environments (Eclipse CDT).

The move of wxGetApp() to the beginning of the macro is only to ensure that a
semicolon is still required after wxIMPLEMENT_APP_NO_MAIN() and the other
macros terminating with it, such as wxIMPLEMENT_APP() itself.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71423 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775