Remove "anyOf" from metricdefinition schema

## What are the schemas that are affected by the issue
- schemas/data/application-closes.schema.json
- schemas/data/bitrate-changes.schema.json
- schemas/data/bounces.schema.json
- schemas/data/buffers.schema.json
- schemas/data/cart-abandons.schema.json
- schemas/data/checkouts.schema.json
- schemas/data/clicks.schema.json
- schemas/data/completes.schema.json
- schemas/data/conversions.schema.json
- schemas/data/drop-before-starts.schema.json
- schemas/data/dropped-frames.schema.json
- schemas/data/errors.schema.json
- schemas/data/feature-usages.schema.json
- schemas/data/federated.schema.json
- schemas/data/first-launches.schema.json
- schemas/data/firstquartiles.schema.json
- schemas/data/impressions.schema.json
- schemas/data/installs.schema.json
- schemas/data/launches.schema.json
- schemas/data/linkclicks.schema.json
- schemas/data/measuredadnotvisible.schema.json
- schemas/data/measuredmuted.schema.json
- schemas/data/measuredwindowinactive.schema.json
- schemas/data/media-segment-views.schema.json
- schemas/data/metricdefinition.schema.json
- schemas/data/midpoints.schema.json
- schemas/data/mirror-pages.schema.json
- schemas/data/non-deliverables.schema.json
- schemas/data/not-sent.schema.json
- schemas/data/opens.schema.json
- schemas/data/pageviews.schema.json
- schemas/data/pauses.schema.json
- schemas/data/poi-entries.schema.json
- schemas/data/poi-exits.schema.json
- schemas/data/product-list-adds.schema.json
- schemas/data/product-list-opens.schema.json
- schemas/data/product-list-removals.schema.json
- schemas/data/product-list-reopens.schema.json
- schemas/data/product-list-views.schema.json
- schemas/data/product-views.schema.json
- schemas/data/progress10.schema.json
- schemas/data/progress95.schema.json
- schemas/data/purchases.schema.json
- schemas/data/resumes.schema.json
- schemas/data/save-for-laters.schema.json
- schemas/data/sends.schema.json
- schemas/data/stall-time.schema.json
- schemas/data/stalls.schema.json
- schemas/data/starts.schema.json
- schemas/data/thirdquartiles.schema.json
- schemas/data/unmeasurableiframe.schema.json
- schemas/data/unmeasurableother.schema.json
- schemas/data/unsubscriptions.schema.json
- schemas/data/upgrades.schema.json
- schemas/data/user-complaints.schema.json
- schemas/data/viewabilityeligibleimpressions.schema.json
- schemas/data/viewablecompletes.schema.json
- schemas/data/viewablefirstquartiles.schema.json
- schemas/data/viewableimpressions.schema.json
- schemas/data/viewablemidpoints.schema.json
- schemas/data/viewablethirdquartiles.schema.json

## What are examples of products that are impacted by the issue
* Adobe Media Analytics

Internal JIRA issue: https://jira.corp.adobe.com/browse/VA-3264

High-level changes:
* The xdm/data/metricdefinition schema contained an "anyOf" statement, which is not supported in AEP. This change is for making the "xdm:unit" field of type string instead of being of type string or null. The null can be represented as empty string.

Start with schemas/data/metricdefinition.schema.json and compare it with the other affected schemas listed in the issue. Update the xdm:unit definition so it is a string rather than a string-or-null anyOf, then confirm the affected schema files no longer contain the unsupported anyOf statement.

Written by the indexing model from the issue text.