Remove unused HAL linker script overrides

The options `MDH_ARM_ROM_START` and `MDH_ARM_ROM_SIZE` override memory
layouts of linker scripts from mcu-driver-reference-platforms-for-arm.
But each example in the SDK comes with its own linker scripts, so there
is no point setting those variables.

Similarly, as the examples do not use the HAL-provided INTERFACE target
`mdh-arm-an552-mps3-startup` for startup file and linker script, we
should not base any target check on this. Fix the target check in the
TF-M firmware update example to use `mdh-arm-an552-mps3` instead, as all
the other examples do.

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