Remove the usage of returns_nonnull to make sure that we'll compile w…

…ith gcc 4.8.x again

Summary:
The issue was exposed by facebook/mysql-5.6#207. The root cause is the fact that gcc 4.8.x doesn't implement a support for `returns_nonnull`. Take a look at https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Function-Attributes.html vs. https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Function-Attributes.html.

For last couple of months we've been operating under the assumption that gcc 4.9.x is the lowest supported platform. We need to go back and internally either revisit this decision or be very explicit about our dependency requirements.

Test Plan:
Optimistic fix, needs to be really verified on the system running gcc 4.8.x.

```
mysqlbuild.sh
```

Reviewers: yoshinorim, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D55641

ha_rocksdb.h

@@ -596,7 +596,6 @@ class ha_rocksdb: public handler
   ulong index_flags(uint inx, uint part, bool all_parts) const;
 
   const key_map * keys_to_use_for_scanning()
-    __attribute__((__returns_nonnull__))
   {
     return &key_map_full;
   }

ha_rocksdb_proto.h

@@ -46,15 +46,15 @@ void request_save_stats();
 
 class Dict_manager;
 Dict_manager *get_dict_manager(void)
-  __attribute__((__returns_nonnull__, __warn_unused_result__));
+  __attribute__((__warn_unused_result__));
 
 class Table_ddl_manager;
 Table_ddl_manager *get_ddl_manager(void)
-  __attribute__((__returns_nonnull__, __warn_unused_result__));
+  __attribute__((__warn_unused_result__));
 
 class Binlog_info_manager;
 Binlog_info_manager *get_binlog_manager(void)
-  __attribute__((__returns_nonnull__, __warn_unused_result__));
+  __attribute__((__warn_unused_result__));
 
 }  // namespace myrocks