x86: doc: Remove stale sections of 64-bit support

There are some sections in current doc saying 64-bit is unsupported.
This apparently is out of date. Remove it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/README.x86

@@ -1160,27 +1160,10 @@ to load a 'u-boot-payload.efi', see below test logs on QEMU.
 
 See README.u-boot_on_efi and README.uefi for details of EFI support in U-Boot.
 
-64-bit Support
---------------
-U-Boot supports booting a 64-bit kernel directly and is able to change to
-64-bit mode to do so. However, U-Boot itself is currently always built
-in 32-bit mode. Some access to the full memory range is provided with
-arch_phys_memset().
-
-The development work to make U-Boot itself run in 64-bit mode has not yet
-been attempted. The best approach would likely be to build a 32-bit SPL
-image for U-Boot, with CONFIG_SPL_BUILD. This could then handle the early CPU
-init in 16-bit and 32-bit mode, running the FSP and any other binaries that
-are needed. Then it could change to 64-bit model and jump to U-Boot proper.
-
-Given U-Boot's extensive 64-bit support this has not been a high priority,
-but it would be a nice addition.
-
 TODO List
 ---------
 - Audio
 - Chrome OS verified boot
-- Building U-Boot to run in 64-bit mode
 
 References
 ----------