This reverts commit cbd6bb1.

<!-- The PR description should answer 2 important questions: -->

### What

When a `comparableRelationship` on a boolean expression type is defined,
but the target model did not have a `filterExpressionType` defined, we
acted as if the `comparableRelationship` was not defined, which is
really confusing.

This PR
a) adds a warning when such a thing that is set up, that turns into an
error when the compat flag is on
b) makes the `boolean_expression_type` in a resolved comparable
relationship optional, so we can tell the difference between "it's
defined but there's nothing there" and "it's not defined"

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

V3_GIT_ORIGIN_REV_ID: 592385b9c942a39ff838130bb72d2e41790095d8

…ression_type` (#1867)

<!-- The PR description should answer 2 important questions: -->

### What

Since #1836 we've kept comparable relationships around, even if they
don't point to anything, so that later lookup errors can tell the
difference between "no relationship was defined" and "the relationship
points to nothing".

This means this model source checking is running where it wasn't before,
which is stopping a few builds generating.

### How

If a comparable relationship does not have a `booleanExpressionType` on
it, skip the checks.

V3_GIT_ORIGIN_REV_ID: 1da8258af0933b42b9d929a2961a4cc19d1bf7bb