fix(query): cast $pull using embedded discriminator schema when discriminator key is set in filter

[vkarpov15]

Fix #14675

Summary

For other update operators, we correctly handle casting embedded discriminator paths when we can infer the embedded discriminator type from the filter. But looks like not for $pull. This PR fixes that.

We should really clean up the duplicated _getSchema() followed by getEmbeddedDiscriminatorPath() calls. Looks like the only reason why we need to call _getSchema() is because of $fullPath, but we need to investigate some more to figure that out.

Examples