remove Loc from protobufs in AST.proto

[cdisselkoen]

Description of changes

Removes source locations from all the protobuf formats in AST.proto. They are not used by the Lean code which consumes them. And, since each Loc includes a full copy of the original source string, they blow up the encoded size of the protobufs significantly.

Breaking change, but protobufs is an experimental feature and this is exactly the kind of change folks should expect while the feature is experimental.

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar language specification.

[john-h-kastner-aws]

Also needs the update in cedar-policy-cli

[cdisselkoen]

Thanks for the reminder, done