cpu/amd/agesa: Remove leftover code

Now that all agesa CPUs are removed this code is unused.

Change-Id: If0c082bbdb09457e3876962fa75725add11cb67c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69118
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/northbridge/amd/agesa/Kconfig

@@ -2,7 +2,6 @@
 
 config NORTHBRIDGE_AMD_AGESA
 	bool
-	default CPU_AMD_AGESA
 
 if NORTHBRIDGE_AMD_AGESA