x86/bugs: Remove duplicate Spectre cmdline option descriptions

Duplicating the documentation of all the Spectre kernel cmdline options
in two separate files is unwieldy and error-prone.  Instead just add a
reference to kernel-parameters.txt from spectre.rst.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Link: https://lore.kernel.org/r/450b5f4ffe891a8cc9736ec52b0c6f225bab3f4b.1719381528.git.jpoimboe@kernel.org