Remove SkipMountingPartitions from vendor libfstab

SkipMountingPartitions() should only be called from a system process
as the config file is in /system_ext.
Remove it from the vendor variant of libfstab.

Bug: 158301941
Test: build hardware/interfaces/boot/1.1/default/boot_control and check symbol
Change-Id: I834183a623c0711dd79b89486fc3fc80f9710801
Merged-In: I834183a623c0711dd79b89486fc3fc80f9710801
(cherry picked from e7cb09d2)