Remove HARNESS_OSSL_PREFIX manipulation in the test harness

Remove HARNESS_OSSL_PREFIX manipulation in the test harness

The aim of HARNESS_OSSL_PREFIX environment variable is to avoid contaminating
TAP producer's output with stanzas that can be interpreted by a TAP producer
by prefixing them with comment;  this can be achieved by processing
the output within the runner instead, as it already does for non-standard
prefixes;  it also has the added benefit of alleviating the need
to reset it for the external tests in order to avoid messing
with their output checks.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28025)