cpu/amd/mtrr/amd_mtrr.c: Remove unused functions

AGESA sets up MTRRs so these functions are now unused.

Change-Id: Ic2bb36d72944ac86c75c163e130f1eb762a7ca37
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58689
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cpu/amd/mtrr.h

@@ -43,7 +43,6 @@
 #include <cpu/x86/msr.h>
 #include <stdint.h>
 
-void amd_setup_mtrrs(void);
 struct device;
 void add_uma_resource_below_tolm(struct device *nb, int idx);