x86: remove 8254 timer texts from Documentation

Commit ecd29476 removed the
"disable_8254_timer" and "enable_8254_timer" kernel parameters from
the kernel but did not remove the references to them from two
files in the Documentation directory: kernel-parameters.txt and
x86/x86_64/boot-options.txt.

This change completes the removal.

Signed-off-by: Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br>
Acked-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>