x86/cpu: Drop unused Kconfig symbol X86_P6_NOP

x86/cpu: Drop unused Kconfig symbol X86_P6_NOP

This symbol was removed in early 2025 but 2 dangling references to it
were missed. Delete them now.

It should be safe to drop the -mtune=generic32 option since gcc 4.3
and later do not cause the problem (see 28f7e66fc1da ("x86: prevent
binutils from being "smart" and generating NOPLs for us")). Also, Arnd
confirmed this with gcc-8 and gcc-15 (see Link:).

Fixes: f388f60ca904 ("x86/cpu: Drop configuration options for early 64-bit CPUs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://patch.msgid.link/20260106014708.991447-1-rdunlap@infradead.org
Link: https://lore.kernel.org/all/c0f0814a-8333-49e1-8e50-740e4c88d94b@app.fastmail.com/