clean up unnecessary outer rust gen-snippet (#5013)

* clean up unneeded outer rust gen-snippet

* update generated rust file

src/idl_gen_rust.cpp

@@ -295,9 +295,6 @@ class RustGenerator : public BaseGenerator {
   bool generate() {
     code_.Clear();
     code_ += "// " + std::string(FlatBuffersGeneratedWarning()) + "\n\n"; 
-    code_ += "#![allow(dead_code)]";
-    code_ += "#![allow(unused_imports)]";
-    code_ += "extern crate flatbuffers;\n";
 
     assert(!cur_name_space_);
 

tests/monster_test_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
-#![allow(dead_code)]
-#![allow(unused_imports)]
-extern crate flatbuffers;
-
 pub mod my_game {
   #![allow(dead_code)]
   #![allow(unused_imports)]

tests/namespace_test/namespace_test1_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
-#![allow(dead_code)]
-#![allow(unused_imports)]
-extern crate flatbuffers;
-
 pub mod namespace_a {
   #![allow(dead_code)]
   #![allow(unused_imports)]

tests/namespace_test/namespace_test2_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
-#![allow(dead_code)]
-#![allow(unused_imports)]
-extern crate flatbuffers;
-
 pub mod namespace_a {
   #![allow(dead_code)]
   #![allow(unused_imports)]