Drop graceful shutdown feature flag

`IPROTO_FEATURE_GRACEFUL_SHUTDOWN` feature flag was dropped in https://github.com/tarantool/tarantool/commit/2e9cbec3091e7c5d6ca1d7dad4305333e70f5876 commit before release, so there is no traces of this flag in Tarantool releases and it is possible that some new meaning would be used for `IPROTO_FEATURE_X = 4`, so we must drop it too to not confuse users.