chore: move the SlackBuild out to the my-slackbuilds repo

The packaging under assets/slackbuild/ had been stale since 0.7.0 and was
deliberately not SBo compliant, carrying a _danix tag and a txz package type
because it was never an SBo submission. It is not what the user builds from, so
it lived here collecting drift: it predated the translations added in 0.23.0 and
would have shipped a package without them.

It now lives in the my-slackbuilds repo under qtmaildir/, rewritten from the SBo
cmake template rather than carried across, at 0.24.0 and building from the
GitHub release tarball rather than a cgit snapshot whose checksum can change
when the tarball is regenerated. That repo has the workflow for bumping it and
an nvchecker stanza that picks up releases from here.

README points at the repo instead of describing a local directory, and keeps the
build invocation so the section still tells a Slackware reader what to do.
CLAUDE.md's release procedure drops the "do not bump this as a side effect"
warning, which has nothing left to warn about.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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