CMake: Remove unused definitions

The definitions removed include:
* `MBED_CONF_SARA4_*`
* `MBED_CONF_GENERIC_AT3GPP_*`
* `MBED_CONF_ALT1250_*`
* `PPP_DEBUG_*`
* `MBED_LFS_*`
* `MEM_ALLOC`
* `MEM_FREE*`
* `NS_USE_EXTERNAL_MBED_TLS`
* `MBED_TRACE_COLOR_THEME`
* `MBED_CONF_GEMALTO_*`
* `MBED_CONF_MCR20A_*`
* `MBED_CONF_S2LP_*`
* `MBED_MPU_CUSTOM`

They are not referenced in the source code.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>