Remove the mount points defined in skip_mount.cfg from ReadDefaultFstab()

The first stage init skips mounting the mount points defined in
skip_mount.cfg, but these mount points still return from
ReadDefaultFstab(). The behavior causes some error logic which
try to access the partition which had been skipped.

After applying the patch. ReadDefaultFstab() will not contain the
skipped mount points.

Bug: 128961335
Test: `fastboot delete-logical-partition product_a`
Test: `fastboot flash system aosp_arm64-userdebug`
Test: `fastboot -w reboot` and boot to home screen
Change-Id: I3156260b5d37647dbecf98ca90601a089bea5c46