Remove unused BorderedText variables from DetectoryActivity

[q3ni]

Hi,
we were reading through how the image labelling was being added to the CameraPreview and it looked like these variables were not used. It looks like this is now handled through MultiBoxTracker and this is redundant.
Thanks!

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[MarkDaoust]

Hi @q3ni,

We can't merge this unless you sign the CLA. Be sure to use the same email address as you used to author the commits.

Thanks.

[q3ni]

@googlebot I signed it!