config: Remove OBERON_BACKEND

The mbedTLS legacy crypto library with Oberon is not available
with the new TF-PSA-Crypto and thus this Kconfig option is removed.

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>

Author: Vge0rge

config/nrfconnect/chip-module/Kconfig.defaults

@@ -418,9 +418,6 @@ endif # CHIP_CRYPTO_PSA
 
 if !CHIP_CRYPTO_PSA
 
-config OBERON_BACKEND
-    default y
-
 config MBEDTLS_AES_C
     default y