xen-netfront: remove leftover call to xennet_tx_buf_gc()

In talk_to_netback(), called earlier from xennet_connect(), queues and
shared rings were just re-initialized, so all this function call could
result in is setting ->broken (again) right away in case any unconsumed
responses were found.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>