revert the dce change wrt externs for now (see #3207)

dce.ml

@@ -56,6 +56,7 @@ let keep_whole_class dce c =
 	|| super_forces_keep c
 	|| (match c with
 		| { cl_path = ([],("Math"|"Array"))} when dce.com.platform = Js -> false
+		| { cl_extern = true }
 		| { cl_path = ["flash";"_Boot"],"RealBoot" } -> true
 		| { cl_path = [],"String" }
 		| { cl_path = [],"Array" } -> not (dce.com.platform = Js)

extra/CHANGES.txt

@@ -22,9 +22,6 @@
 	cpp : fixed issue with the side-effect handler
 	cpp : fixed issue with NativeArray in --no-inline mode
 	php : fixed issue with invalid references for closures in for-loops
-	all : fixed __init__ methods of extern classes being generated even if the class is not used (but imported).
-	      this change may break existing code that relies on that bug. to force extern class __init__ generation,
-	      use the @:keepInit class-level metadata
 
 	General improvements and optimizations: