Haiku: Remove remnants of past WIP platform port

The Haiku platform port was never finalized, and moved to a separate repo in
Godot 3.2 days: https://github.com/godotengine/godot-platform-haiku

Sadly it didn't garner more interest there and is bitrotting. It was never
ported to Godot 4 so the bits of Haiku support left in Mono aren't useful.

drivers/gl_context/SCsub

@@ -2,7 +2,7 @@
 
 Import("env")
 
-if env["platform"] in ["haiku", "macos", "windows", "linuxbsd"]:
+if env["platform"] in ["macos", "windows", "linuxbsd"]:
     # Thirdparty source files
     thirdparty_dir = "#thirdparty/glad/"
     thirdparty_sources = [