mtd: Remove a debug trace in mtdpart.c

Commit 2b6f0090 ("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: 2b6f0090

 ("mtd: Check add_mtd_device() ret code")
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>