apps/lib/apps.c: remove HARNESS_OSSL_PREFIX envvar handling

apps/lib/apps.c: remove HARNESS_OSSL_PREFIX envvar handling

As the environment variable is no longer used by the test harness,
its support can be removed from the openssl command, along
with the possible unintended consequences of prefixing the standard
output of the openssl command with an arbitrary string set
in the environment.

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)