Remove explicit link to MBEDTLS_CONFIG_FILE
Remove explicit link to MBEDTLS_CONFIG_FILE

This symbol is not declared in our code, so trying to explicitly
link to it causes a doxygen error.

Signed-off-by: Bence Szépkúti <f5a052dc170d15117fd347c2abf2400eb6ae5243@arm.com>

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