docs: driver-api: remove a duplicated index entry

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 928123f08c09f9921f9736405ce18b566f0e6165
	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 5503737f4de2c3a67a2499ba08238db5f4c3425d
	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: 928123f08c09 ("docs: ipmb: place it at driver-api and convert to ReST")
Fixes: 5503737f4de2 ("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>