The function p2m_index is defined in the p2m.c file, but not called
elsewhere, so remove this unused function.

arch/x86/xen/p2m.c:137:24: warning: unused function 'p2m_index'.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3557Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230105090141.36248-1-jiapeng.chong@linux.alibaba.comSigned-off-by: Juergen Gross <jgross@suse.com>