Remove the barely used macro assemblers after merging their contents to the base

runtime/vm/flow_graph_compiler_arm.cc

Index: runtime/vm/flow_graph_compiler_arm.cc
===================================================================
--- runtime/vm/flow_graph_compiler_arm.cc	(revision 19430)
+++ runtime/vm/flow_graph_compiler_arm.cc	(working copy)
@@ -216,7 +216,7 @@
                          0);  // No token position.
   }
   __ Comment("Enter frame");
-  AssemblerMacros::EnterDartFrame(assembler(), (StackSize() * kWordSize));
+  __ EnterDartFrame((StackSize() * kWordSize));
 }
 
 
@@ -306,7 +306,7 @@
                                             0);  // No registers.
         }
         // The noSuchMethod call may return.
-        AssemblerMacros::LeaveDartFrame(assembler());
+        __ LeaveDartFrame();
         __ Ret();
       } else {
         __ Stop("Wrong number of arguments");