Remove unnecessary explicit MBEDTLS_SSL_PROTO_TLS1_2
dependency if TLS 1.2 version is forced or a TLS 1.2
cipher suite is forced (as TLS 1.2 cipher suites are
available if and only if TLS 1.2 is enabled and
cipher suite availability is check automatically).
Signed-off-by: Ronald Cron <ronald.cron@arm.com>