Move Haiku platform port to external repository

The Haiku port now resides at:
https://github.com/godotengine/godot-haiku-platform

While we're happy to support as many Free and Open Source platforms as we can,
we need to put the main focus on the platforms that we can reasonably maintain,
and for Haiku we have been lacking maintainers ever since the port was first
merged in 2015.

The Haiku code has not been compiling and much less working at least since the
release of Godot 3.0, and while some attempts have been made at fixing things,
it's still not functional today in the `3.2` branch (and much less in `master`,
understandably).

Having it in an external repository should hopefully enable Haiku contributors
to fix issues in their own time, and possibly tag versions compatible with
past Godot releases once they are ready.

doc/classes/OS.xml

@@ -205,7 +205,7 @@
 			<return type="String">
 			</return>
 			<description>
-				Returns the name of the host OS. Possible values are: [code]"Android"[/code], [code]"Haiku"[/code], [code]"iOS"[/code], [code]"HTML5"[/code], [code]"OSX"[/code], [code]"Server"[/code], [code]"Windows"[/code], [code]"UWP"[/code], [code]"X11"[/code].
+				Returns the name of the host OS. Possible values are: [code]"Android"[/code], [code]"iOS"[/code], [code]"HTML5"[/code], [code]"OSX"[/code], [code]"Server"[/code], [code]"Windows"[/code], [code]"UWP"[/code], [code]"X11"[/code].
 			</description>
 		</method>
 		<method name="get_process_id" qualifiers="const">