mtd: Remove a debug trace in mtdpart.c

[ Upstream commit bda2ab56356b9acdfab150f31c4bac9846253092 ]

Commit 2b6f0090a333 ("mtd: Check add_mtd_device() ret code") contained
a leftover of the debug session that led to this bug fix. Remove this
pr_info().

Fixes: 2b6f0090a333 ("mtd: Check add_mtd_device() ret code")
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>