Drop unused Assembler::set_external_target_at() method.

src/arm/assembler-arm.h

Index: src/arm/assembler-arm.h
diff --git a/src/arm/assembler-arm.h b/src/arm/assembler-arm.h
index 137dd73e992549c5b3069b218b5ef8a2f5480fc7..afb0c62787a4ca5e22795179a0d0cb2079e97a7a 100644
--- a/src/arm/assembler-arm.h
+++ b/src/arm/assembler-arm.h
@@ -803,11 +803,6 @@ class Assembler : public AssemblerBase {
   inline static void deserialization_set_special_target_at(
       Address constant_pool_entry, Address target);
 
-  // This sets the branch destination (which is in the constant pool on ARM).
-  // This is for calls and branches to runtime code.
-  inline static void set_external_target_at(Address constant_pool_entry,
-                                            Address target);
-
   // Here we are patching the address in the constant pool, not the actual call
   // instruction.  The address in the constant pool is the same size as a
   // pointer.