rgw/rgw_metadata.cc: delete md_log (RGWMetadataLog) in destructor

Call delete on md_log in the destructor.

CID 1054826 (#1 of 1): Resource leak in object (CTOR_DTOR_LEAK)
 1. alloc_new: Allocating memory by calling "new RGWMetadataLog(_cct, _store)".
 2. var_assign: Assigning: "this->md_log" = "new RGWMetadataLog(_cct, _store)".
 3. ctor_dtor_leak: The constructor allocates field "md_log" of
    "RGWMetadataManager" but the destructor and whatever functions it calls
    do not free it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>