rgw/rgw_rest_sts: return local variable w/o std::move()

rgw/rgw_rest_sts: return local variable w/o std::move()

in C++17, the C++ standard ensures that the return value in this case
can be optimized with copy elision, adding std::move() is not necessary,
and prevents copy elision from happening.

this change also silences the warning of:

../src/rgw/rgw_rest_sts.cc: In member function 'std::unique_ptr<rgw::sal::RGWOIDCProvider> rgw::auth::sts::WebTokenEngine::get_provider(const DoutPrefixProvider*, const string&, const string&) const':
../src/rgw/rgw_rest_sts.cc:92:19: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
   92 |   return std::move(provider);
      |          ~~~~~~~~~^~~~~~~~~~
../src/rgw/rgw_rest_sts.cc:92:19: note: remove 'std::move' call

Signed-off-by: Kefu Chai <kchai@redhat.com>