This commit removes the definition of the API function

`mbedtls_platform_set_calloc_free()`

from `library/platform.c` in case the macros

`MBEDTLS_PLATFORM_CALLOC_MACRO`
`MBEDTLS_PLATFORM_FREE_MACRO`

for compile time configuration of calloc/free are set.

This is in line with the corresponding header `mbedtls/platform.h`
which declares `mbedtls_platform_set_calloc_free()` only if
`MBEDTLS_PLATFORM_{CALLOC/FREE}_MACRO` are not defined.

Fixes #1642.