x86/of: Remove unused early_init_dt_add_memory_arch()

Recently objtool started complaining about dead code in the object files,
in particular

vmlinux.o: warning: objtool: early_init_dt_scan_memory+0x191: unreachable instruction

when CONFIG_OF=y.

Indeed, early_init_dt_scan() is not used on x86 and making it compile (with
help of CONFIG_OF) will abrupt the code flow since in the middle of it
there is a BUG() instruction.

Remove the pointless function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221124184824.9548-1-andriy.shevchenko@linux.intel.com