Remove cipher_test_positive, duplicated as cipher_test_encrypt
Remove cipher_test_positive, duplicated as cipher_test_encrypt

cipher_test_positive was never compiled due to a syntax error in the
BEGIN_CASE magic comment. It has now been duplicated as
cipher_test_encrypt. Remove the copy that was never compiled.

ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls