Remove unused FLAG_always_megamorphic_calls.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index 87c418f2a82b887fd5640662678cd5a446270dff..b98a3fc85611f4d2d4ad813e63ed350e31074bcd 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -84,7 +84,6 @@ static void PrecompilationModeHandler(bool value) {
     FLAG_inlining_constant_arguments_min_size_threshold = 30;
 
     FLAG_background_compilation = false;
-    FLAG_always_megamorphic_calls = true;
     FLAG_fields_may_be_reset = true;
     FLAG_interpret_irregexp = true;
     FLAG_lazy_dispatchers = false;
@@ -1196,12 +1195,6 @@ void FlowGraphCompiler::GenerateInstanceCall(
                                deopt_id, token_pos, locs);
     return;
   }
-  if (FLAG_always_megamorphic_calls) {
-    EmitMegamorphicInstanceCall(ic_data_in, argument_count,
-                                deopt_id, token_pos, locs,
-                                CatchClauseNode::kInvalidTryIndex);
-    return;
-  }
   ASSERT(!ic_data.IsNull());
   if (is_optimizing() && (ic_data_in.NumberOfUsedChecks() == 0)) {
     // Emit IC call that will count and thus may need reoptimization at