mesa: Remove last BEOS checks

* Goodbye BeOS, we hardly knew thee
* As BeOS was gcc2 only, there was little chance
  of this being useful.
* Doesn't effect Haiku in any meaningful way

Reviewed-by: Brian Paul <brianp@vmware.com>

scons/gallium.py

@@ -295,8 +295,6 @@ def generate(env):
             cppdefines += ['_DEBUG']
     if platform == 'windows':
         cppdefines += ['PIPE_SUBSYSTEM_WINDOWS_USER']
-    if platform == 'haiku':
-        cppdefines += ['BEOS_THREADS']
     if env['embedded']:
         cppdefines += ['PIPE_SUBSYSTEM_EMBEDDED']
     if env['texture_float']: