Remove lifetime specifier on table getter methods (#4949)

With the old-style code, the test fails with a borrow-checker error:

```
  #[inline]
  pub fn name(&'a self) -> &'a str {
    self._tab.get::<flatbuffers::ForwardsUOffset<&str>>(Monster::VT_NAME, None).unwrap()
  }
```

```
error[E0597]: `e` does not live long enough
   --> tests/integration_test.rs:273:57
    |
273 |         let enemy_of_my_enemy = monster.enemy().map(|e| e.name());
    |                                                         ^      - `e` dropped here while still borrowed
    |                                                         |
    |                                                         borrowed value does not live long enough
274 |         assert_eq!(enemy_of_my_enemy, Some("Fred"));
275 |     }
    |     - borrowed value needs to live until here
```

src/idl_gen_rust.cpp

@@ -1263,7 +1263,7 @@ class RustGenerator : public BaseGenerator {
 
       GenComment(field.doc_comment, "  ");
       code_ += "  #[inline]";
-      code_ += "  pub fn {{FIELD_NAME}}(&'a self) -> {{RETURN_TYPE}} {";
+      code_ += "  pub fn {{FIELD_NAME}}(&self) -> {{RETURN_TYPE}} {";
       code_ += "    {{FUNC_BODY}}";
       code_ += "  }";