Deprecate SPT threading support on NonStop.

Deprecate SPT threading support on NonStop.

This fix removes explicit support for the SPT threading model in configurations.
This also reverts commit f63e1b48ac893dd6110452e70ed08f191547cd89 that were
required for SPT but broke other models.

Fixes: #22798
Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22807)