Exclude not used schemas

Created a list of all used schemas, and a few being added for use in reviews
now. Exclude all other schemas.

Developers if they wish to use a new schema will have to add it to this
list and rerun update_schemas.py.

Size of obmc-phosphor-image-witherspoon-<date>.rootfs.squashfs-xz:
Before 2020.4: 19451904
With 2020.4, before this commit: 19492864
With this commit: 19275776

Tested: Validator passes.
Change-Id: I23272049aa38c51ac7af0d3b0f5b4641a349a487
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>