Remove: BeOS support (deprecated by Haiku)

In 10 years there is no commit to change how BeOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), BeOS is no longer
support. SDL2 suggests to use Haiku instead of BeOS.

README.md

@@ -152,7 +152,6 @@ OpenTTD has been ported to several platforms and operating systems. It should
 not be very difficult to port it to a new platform. The currently working
 platforms are:
 
-- BeOS (SDL or Allegro)
 - DOS (Allegro)
 - FreeBSD (SDL)
 - Linux (SDL or Allegro)
@@ -546,10 +545,6 @@ However, for the first build one has to do a '`./configure`' first.
 To make a universal binary type '`./configure --enabled-universal`'
 instead of '`./configure`'.
 
-### BeOS:
-
-Use '`make`', but do a '`./configure`' before the first build.
-
 ### OS/2:
 
 A comprehensive GNU build environment is required to build the OS/2 version.