Docs/admin-guide: Remove pmf leftover reference from the index

pmf.rst was removed by the commit 2fd66f7 ("platform/x86/amd/pmf:
Remove update system state document") but the reference in the
admin-guide index remained in place which triggers this warning:

Documentation/admin-guide/index.rst:75: WARNING: toctree contains
reference to nonexisting document 'admin-guide/pmf'

Remove pmf also from the index to avoid the warning.

Reported-by: Stephen Rothwell <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ilpo Järvinen <[email protected]>

Author: ij-intel

Documentation/admin-guide/index.rst

@@ -121,7 +121,6 @@ configure specific aspects of kernel behavior to your liking.
    parport
    perf-security
    pm/index
-   pmf
    pnp
    rapidio
    RAS/index