Remove the flash sample

Adobe have announced the official EOL for flash is 2020 so we're
now at the point where support for flash just isn't interesting
any more.

It doesn't make sense to support it for the upcoming 3.2.x release
series, and the sample .swf files are lacking source code.

Closes https://github.com/wxWidgets/wxWidgets/pull/1427

Closes #15886.

docs/doxygen/mainpages/samples.h

@@ -307,23 +307,6 @@ wxProcess::Exists().
 
 @sampledir{exec}
 
-@section page_samples_flash Flash Sample
-
-The flash sample demonstrates embedding of Adobe Flash into a wxWidgets
-program. Currently it only works under Windows as it uses the Flash ActiveX
-control to achieve this but we hope to be able to extend it to also work under
-other platforms in the future. The sample also currently requires Microsoft
-Visual C++ compiler as it uses COM support extensions specific to this
-compiler.
-
-The sample comes with 2 Flash files (SWF), showing a simple Flash animation
-which can be controlled using the "Play", "Stop" and "Back"/"Forward" buttons
-in the sample as well as a Flash form which shows how Flash and wxWidgets
-program can exchange data: calling "GetText" function without arguments returns
-the text of the text control defined inside Flash and calling "SetText" with an
-argument sets the control contents to the given string. Finally clicking on the
-button generates an event which is caught by the C++ program.
-
 @section page_samples_font Font Sample
 
 The font sample demonstrates wxFont,