soc: silabs: Drop useless SOC_PART_NUMBER

Variable CONFIG_SOC_PART_NUMBER is only used in CMakeLists.txt of
hal_silabs. In fact, this variable can be easily calculated from CONFIG_SOC
by changing lower case in upper case.

So, let's drop this useless variable.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>