soc: silabs: drop useless defconfigs

Currently, some soc subdirectories contains Kconfig.defconfig.<soc-name>
and Kconfig.defconfig. However:
  - Kconfig.defconfig.<soc-name> is included unconditionally by
    Kconfig.defconfig
  - each subdirectory only contains one SoC

So, it does not make sense to keep these two files.

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