Secure block discard/trim was fixed by:
https://github.com/CyanogenMod/android_external_f2fs-tools/commit/6b39b93d83ab4ccb1913ecbc7a5a3e16766a011f

Once fixed, formatting using discard/trim takes a LONG time.  Other TWRP
formatting methods are already insecure.  It doesn't make sense for this
one type of partition to have secure wipe when none of the others do.

Change-Id: Ib5ad5a722895b0e0c7aa4e3af98b705e2574b762