Unregister error message key on library unload

Unregister error message key on library unload

Revision fcdd2de1 added the K5_KEY_GSS_KRB5_ERROR_MESSAGE key, and
registered it in the gssapi library initialization routine, but
did not unregister it in the libary finalization routine.
When the library is unloaded and reloaded in the same process,
this leads to an assertion failure, since we check that
destructors_set[keynum] is zero (no destructor set) when registering
a key in util/support/threads.c.
Unregister the key on library cleanup to resolve the error.

(cherry picked from commit 4ab584c830024757cc628b1783dde6220a9fec6d)

ticket: 7353
version_fixed: 1.10.4
status: resolved