mtd: lpc32xx: drop bitflip_threshold initialization

These drivers don't need to explicitly initialize their bitflip
thresholds. The comment is no longer correct, since nand_scan_tail()
performs this initialization as of the following commit:

    commit ea3b2ea2
    Author: Shmulik Ladkani <shmulik@jungo.com>
    Date:   Fri Jun 8 18:29:06 2012 +0300

        mtd: nand: initialize bitflip_threshold prior to BBT scanning

(It seems there were some parallel efforts on writing/submitting these
drivers, and Shmulik's bug fix.)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Roland Stigge <stigge@antcom.de>