common: remove unused attribute

common: remove unused attribute

This resolves an obnoxious compiler warning:

    [79/768] Building CXX object src/mgr/CMakeFiles/ceph-mgr.dir/__/ceph_mgr.cc.o
    In file included from /run/host/home/sepian/ceph/src/global/global_init.h:23,
                     from /run/host/home/sepian/ceph/src/ceph_mgr.cc:27:
    /run/host/home/sepian/ceph/src/common/ceph_context.h:153:42: warning: ‘maybe_unused’ attribute ignored [-Wattributes]
      153 |   [[maybe_unused]] std::unique_ptr<char> _ex_handler;
          |                                          ^~~~~~~~~~~

Fixes: 39af0bc23b3ae2063f8e846b0ff522bafa0f5c27
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>