cpu/qemu-x86/cache_as_ram_bootblock: drop duplicated post code

Before the bootblock stage starts setting up the CAR mode, it sends
`POST_BOOTBLOCK_CAR` POST code. However, before the definition for
`POST_BOOTBLOCK_CAR` was introduced in the commit
0d34a50a36 , the value `0x20` was used.
At that point, `0x20` means "entry into CAR mode" and `0x21` means
"the cache memory region is cleared". Right now we are sending the
same POST code twice, which makes no sense.

So we can do the following (todo: drop me after we decided which one is
more appropriate):

1) Drop it (current patchset does exactly that)
2) Introduce POST code similar to POST_SOC_CLEARING_CAR and use it
before the cache memory region is cleared.

Change-Id: I5d9014c788abdf5a4338c9e199138d1e514450b3
Signed-off-by: Alexander Goncharov <chat@joursoir.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73744
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

src/cpu/qemu-x86/cache_as_ram_bootblock.S

@@ -29,8 +29,6 @@ cache_as_ram:
 	xorl	%eax, %eax
 	rep	stosl
 
-	post_code(0x21)
-
 #if defined(__x86_64__)
 	/*
 	 * Copy page tables to final location in DRAM. This prevents some strange