templates: Remove AN552-specific macros and checks

Each example's template should be generic and not containing anything
specific to a given target platform. Remove the following items that
are specific to `mdh-arm-an552-mps3`:
* `LAN91C111_RFS_MULTICAST_SUPPORT`: multicast support is not needed
* `DEVICE_SERIAL_ASYNCH`: its definition has been added to the `mcu-
driver-reference-platforms-for-arm` repository which is the place to
declare what HAL features are supported and unsupported
* target check inside `set_linker_script()`: now that linker scripts
`armclang.sct` and `gcc.ld` are generated from templates, the target
check no longer makes sense.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>