Remove GPL-add-ons build machinery.

No longer used by anything (it appears some old kernel drivers that
are not included in the build might use it, but I don't know why
they need it -- we already ship GPL'd code [libntfs] for the kernel
in the default build unconditionally.)

Briefly discussed with js a few weeks back.

build/jam/BuildFeatures

@@ -677,10 +677,5 @@ if $(TARGET_PACKAGING_ARCH) = x86_64 {
 	}
 }
 
-# GPL add-ons
-if $(HAIKU_INCLUDE_GPL_ADDONS) = 1 {
-	EnableBuildFeatures gpl ;
-}
-
 # ATA Drivers + Bus
 EnableBuildFeatures ata ;

build/jam/MiscRules

@@ -63,15 +63,6 @@ rule SetupFeatureObjectsDir feature
 	;
 }
 
-rule SubIncludeGPL
-{
-	# SubInclude rule that can be used to conditionally include GPL licensed
-	# add-ons
-	if $(HAIKU_INCLUDE_GPL_ADDONS) = 1 {
-		SubInclude $(1) ;
-	}
-}
-
 
 # pragma mark - MakeLocate variants