MNT: Remove unused legacy spectral.cu

[mani-builds]

This removes the long-dead cpp/src/spectral/spectral.cu file and its last remaining reference in the top-level CMakeLists.txt.

The file has been unused since the spectral clustering refactor in 2021 and is no longer compiled or required.

Closes #7538

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[csadorf]

@mani-builds Please refrain from merging origin/main unless there are particular updates you need to integrate from the upstream branch or our bot warns explicitly that the PR branch is too far behind. This is to reduce CI churn on our end. Thank you.

[mani-builds]

@mani-builds Please refrain from merging origin/main unless there are particular updates you need to integrate from the upstream branch or our bot warns explicitly that the PR branch is too far behind. This is to reduce CI churn on our end. Thank you.

Understood, thanks for the feedback.

[csadorf]

/ok to test 8688a02

[aamijar]

/ok to test 7a33489

[aamijar]

/ok to test c99ec1a

[aamijar]

/ok to test 2526b98

[aamijar]

/ok to test e16b8f3

[aamijar]

/merge