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>

soc/silabs/silabs_s1/efm32gg11b/Kconfig.defconfig

@@ -10,6 +10,8 @@ config NUM_IRQS
 	# must be >= the highest interrupt number used
 	default 68
 
-rsource "Kconfig.defconfig.efm32gg11b"
+config GPIO_GECKO
+	default y
+	depends on GPIO || LOG_BACKEND_SWO
 
 endif # SOC_SERIES_EFM32GG11B

soc/silabs/silabs_s1/efm32gg11b/Kconfig.defconfig.efm32gg11b

@@ -1,8 +0,0 @@
-# Silicon Labs EFM32GG-STK3701A platform configuration options
-# Copyright (c) 2019 Interay Solutions B.V.
-# Copyright (c) 2019 Oane Kingma
-# SPDX-License-Identifier: Apache-2.0
-
-config GPIO_GECKO
-	default y
-	depends on GPIO || LOG_BACKEND_SWO

soc/silabs/silabs_s1/efm32gg12b/Kconfig.defconfig

@@ -9,6 +9,8 @@ config NUM_IRQS
 	# must be >= the highest interrupt number used
 	default 68
 
-rsource "Kconfig.defconfig.efm32gg12b"
+config GPIO_GECKO
+	default y
+	depends on GPIO || LOG_BACKEND_SWO
 
 endif # SOC_SERIES_EFM32GG12B

soc/silabs/silabs_s1/efm32gg12b/Kconfig.defconfig.efm32gg12b

@@ -1,7 +0,0 @@
-# Silicon Labs EFM32GG12B (Giant Gecko) platform configuration options
-# Copyright (c) 2023 Antmicro <www.antmicro.com>
-# SPDX-License-Identifier: Apache-2.0
-
-config GPIO_GECKO
-	default y
-	depends on GPIO || LOG_BACKEND_SWO

soc/silabs/silabs_s2/efr32bg22/Kconfig.defconfig

@@ -13,6 +13,7 @@ config PM
 	select COUNTER
 	select UART_INTERRUPT_DRIVEN
 
-rsource "Kconfig.defconfig.efr32bg22"
+config GPIO_GECKO
+	default y
 
 endif # SOC_SERIES_EFR32BG22

soc/silabs/silabs_s2/efr32bg22/Kconfig.defconfig.efr32bg22

@@ -1,7 +0,0 @@
-# Silicon Labs EFR32BG22 (Blue Gecko) MCU configuration options
-
-# Copyright (c) 2021 Sateesh Kotapati
-# SPDX-License-Identifier: Apache-2.0
-
-config GPIO_GECKO
-	default y

soc/silabs/silabs_s2/efr32bg27/Kconfig.defconfig

@@ -12,6 +12,7 @@ config NUM_IRQS
 config PM
 	select COUNTER
 
-rsource "Kconfig.defconfig.efr32bg27"
+config GPIO_GECKO
+	default y
 
 endif # SOC_SERIES_EFR32BG27

soc/silabs/silabs_s2/efr32bg27/Kconfig.defconfig.efr32bg27

@@ -1,7 +0,0 @@
-# Silicon Labs EFR32BG27 (Blue Gecko) MCU configuration options
-
-# Copyright (c) 2023 Antmicro <www.antmicro.com>
-# SPDX-License-Identifier: Apache-2.0
-
-config GPIO_GECKO
-	default y

soc/silabs/silabs_s2/efr32mg21/Kconfig.defconfig

@@ -10,6 +10,8 @@ config NUM_IRQS
 	# must be >= the highest interrupt number used
 	default 61
 
-rsource "Kconfig.defconfig.efr32mg21"
+config GPIO_GECKO
+	default y
+	depends on GPIO || LOG_BACKEND_SWO
 
 endif # SOC_SERIES_EFR32MG21

soc/silabs/silabs_s2/efr32mg21/Kconfig.defconfig.efr32mg21

@@ -1,8 +0,0 @@
-# Silicon Labs EFR32MG21 (Mighty Gecko) MCU configuration options
-
-# Copyright (c) 2020 TriaGnoSys GmbH
-# SPDX-License-Identifier: Apache-2.0
-
-config GPIO_GECKO
-	default y
-	depends on GPIO || LOG_BACKEND_SWO