maint: remove extraneous "const" attribute from return type

Summary:
The "const" attribute does not make sense on a return type,
and provokes a warning/error from gcc -W -Wextra.

Test Plan:
  Run "make EXTRA_CXXFLAGS='-W -Wextra'" and see fewer errors.

Reviewers: ljin, sdong, igor.sugak, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33759

db/c.cc

@@ -584,22 +584,22 @@ int rocksdb_backup_engine_info_count(const rocksdb_backup_engine_info_t* info) {
   return static_cast<int>(info->rep.size());
 }
 
-const int64_t rocksdb_backup_engine_info_timestamp(
+int64_t rocksdb_backup_engine_info_timestamp(
     const rocksdb_backup_engine_info_t* info, int index) {
   return info->rep[index].timestamp;
 }
 
-const uint32_t rocksdb_backup_engine_info_backup_id(
+uint32_t rocksdb_backup_engine_info_backup_id(
     const rocksdb_backup_engine_info_t* info, int index) {
   return info->rep[index].backup_id;
 }
 
-const uint64_t rocksdb_backup_engine_info_size(
+uint64_t rocksdb_backup_engine_info_size(
     const rocksdb_backup_engine_info_t* info, int index) {
   return info->rep[index].size;
 }
 
-const uint32_t rocksdb_backup_engine_info_number_files(
+uint32_t rocksdb_backup_engine_info_number_files(
     const rocksdb_backup_engine_info_t* info, int index) {
   return info->rep[index].number_files;
 }

db/compaction.h

@@ -92,7 +92,7 @@ class Compaction {
   // input level.
   // REQUIREMENT: "compaction_input_level" must be >= 0 and
   //              < "input_levels()"
-  std::vector<FileMetaData*>* const inputs(size_t compaction_input_level) {
+  std::vector<FileMetaData*>* inputs(size_t compaction_input_level) {
     assert(compaction_input_level < inputs_.size());
     return &inputs_[compaction_input_level].files;
   }

include/rocksdb/c.h

@@ -135,19 +135,19 @@ extern const rocksdb_backup_engine_info_t* rocksdb_backup_engine_get_backup_info
 extern int rocksdb_backup_engine_info_count(
     const rocksdb_backup_engine_info_t* info);
 
-extern const int64_t rocksdb_backup_engine_info_timestamp(
+extern int64_t rocksdb_backup_engine_info_timestamp(
     const rocksdb_backup_engine_info_t* info,
     int index);
 
-extern const uint32_t rocksdb_backup_engine_info_backup_id(
+extern uint32_t rocksdb_backup_engine_info_backup_id(
     const rocksdb_backup_engine_info_t* info,
     int index);
 
-extern const uint64_t rocksdb_backup_engine_info_size(
+extern uint64_t rocksdb_backup_engine_info_size(
     const rocksdb_backup_engine_info_t* info,
     int index);
 
-extern const uint32_t rocksdb_backup_engine_info_number_files(
+extern uint32_t rocksdb_backup_engine_info_number_files(
     const rocksdb_backup_engine_info_t* info,
     int index);