Remove old useless function from benchmark
Remove old useless function from benchmark

This no longer makes sense since pre-computed multiples of the base
point are now static. The function was not doing anything since `grp.T`
was set to `NULL` when exiting `ecp_mul_comb()` anyway.

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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