docs: driver-api: remove a duplicated index entry

The ipmb file was added twice at index.rst. That
sounds to be because the same patch was applied twice,
via different git trees:

	commit f6ae22d
	Author: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
	Commit: Jonathan Corbet <corbet@lwn.net>

	    docs: ipmb: place it at driver-api and convert to ReST

	commit ac499fb
	Author: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
	Commit: Corey Minyard <cminyard@mvista.com>

	    docs: ipmb: place it at driver-api and convert to ReST

With Sphinx 4.0.0 development tree, a new warning is produced
due to that:

	.../Documentation/driver-api/index.rst:14: WARNING: duplicated entry found in toctree: driver-api/ipmb

The fix is trivial: just drop the duplicated line.

Fixes: f6ae22d ("docs: ipmb: place it at driver-api and convert to ReST")
Fixes: ac499fb ("docs: ipmb: place it at driver-api and convert to ReST")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/623fb26a8409a7b002e45bdbb6f517ac08fd508a.1602753121.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mchehab

Documentation/driver-api/index.rst

@@ -79,7 +79,6 @@ available subsections can be seen below.
    console
    dcdbas
    eisa
-   ipmb
    isa
    isapnp
    io-mapping