Fix nightly Debian 12/13 Dockerfiles

[kkebo]

These Dockerfiles contain invalid instructions copied from swift-ci Dockerfiles, so building the images will always fail.

This PR removes them.

[kkebo]

Building nightly-6.4.x/debian/13/Dockerfile is failing in CI, but it seems to be an expected behavior since https://download.swift.org/swift-6.4.x-branch/debian13 does not exist yet.

0.765 export HOME='/root'
0.765 export PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
0.765 export PWD='/'
0.765 export SWIFT_PLATFORM='debian13'
0.765 export SWIFT_PREFIX='/opt/swift/6.4'
0.765 export SWIFT_SIGNING_KEY='E813C892820A6FA13755B268F167DF1ACF9CE069'
0.765 export SWIFT_VERSION='6.4'
0.765 export SWIFT_WEBROOT='https://download.swift.org/swift-6.4.x-branch/debian13'
1.192 export HOME='/root'
1.192 export PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
1.192 export PWD='/'
1.192 export SWIFT_PLATFORM='debian13'
1.192 export SWIFT_PREFIX='/opt/swift/6.4'
1.192 export SWIFT_SIGNING_KEY='E813C892820A6FA13755B268F167DF1ACF9CE069'
1.192 export SWIFT_VERSION='6.4'
1.192 export SWIFT_WEBROOT='https://download.swift.org/swift-6.4.x-branch/debian13'
1.997 curl: (22) The requested URL returned error: 404
2.043 curl: (22) The requested URL returned error: 404

[kkebo]

In any case, this PR change isn't the cause.

[kkebo]

This CI failure is expected because the nightly-6.4.x toolchain is not yet distributed for Debian 13.

[FAILED] - nightly-6.4.x/debian/13/Dockerfile

• https://github.com/swiftlang/swift-org-website/blob/main/_data/builds/swift-6_4_x-branch/debian13.yml
• https://github.com/swiftlang/swift-org-website/blob/main/api/v1/install/dev/6.4.x/debian13.json