RISC-V: Drop unused SOC_CANAAN

All users of SOC_CANAAN are now gone and it is not user-visible, remove it.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/Kconfig.socs

@@ -76,10 +76,6 @@ config ARCH_CANAAN
 	help
 	  This enables support for Canaan Kendryte series SoC platform hardware.
 
-config SOC_CANAAN
-	def_bool SOC_CANAAN_K210
-	depends on ARCH_CANAAN
-
 config SOC_CANAAN_K210
 	bool "Canaan Kendryte K210 SoC"
 	depends on !MMU && ARCH_CANAAN