ARM: dts: msm: Override vendor node entires in fstab for MSM8996ProAU

Override the property fsmgr_flags by adding the options verify and
slotselect for the vendor node in the fstab so that A/B is enabled and the
partition is verified by the device mapper target dm-verity. Also delete
the system node from the fstab as it is already mounted by the kernel.

Change-Id: Ib65f1d261c95b995f3999ba24517a69488346ca9
Signed-off-by: Binoy Jayan <bjayan@codeaurora.org>