[18.10] Remove Mattermost for SLES-12.5

What does this MR do and why?

Remove Mattermost for SLES-12.5

• Removes Mattermost from SLES 12.5 to reduce the total files shipped in the package due to limitations in the RPM 4.11 file header specification.

Related #9716 (closed)

Changelog: removed

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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