Remove mateuszc@google.com from tests/OWNERS

This suggested change is automatically generated based on group
memberships and affiliations.
If this change is unnecessary or in error, vote CR -1 and the bot
will abandon it. Vote CR +1/2 to approve this change.

See the owner's recent activity for context:
https://android-review.googlesource.com/q/mateuszc@google.com

To report an issue, file a bug in the Infra>Codereview component.

Change-Id: I0c1c61d68c098cb91f1c9721e8b1902903a0e1a2