[SPARK-27796][MESOS] Remove obsolete spark-mesos Dockerfile example

## What changes were proposed in this pull request?

Remove obsolete spark-mesos Dockerfile example. This isn't tested and apparently hasn't been updated in 4 years.

## How was this patch tested?

N/A

Closes apache#24667 from srowen/SPARK-27796.

Authored-by: Sean Owen <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: srowen