selftests: xsk: Remove Tx synchronization resources

Tx thread needs to be started after the Rx side is fully initialized so
that packets are not xmitted until xsk Rx socket is ready to be used.

It can be observed that atomic variable spinning_tx is not checked from
Rx side in any way, so thread_common_ops can be modified to only address
the spinning_rx. This means that spinning_tx can be removed altogheter.

signal_tx_condition is never utilized, so simply remove it.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-11-maciej.fijalkowski@intel.com