Build/Test Tools: Remove obsolete "reusable-performance.yml" workflow file

[mukesh27]

In ticket #62221 / [59749], the performance tests were updated to utilize a new parallelized workflow.

Following this update, the use of the reusable-performance.yml GitHub Actions workflow file was completely phased out and removed from the active CI pipeline. However, the physical file itself was left behind in the .github/workflows/ directory.

To prevent any future confusion or misunderstanding regarding the active configuration files, this obsolete workflow file should be permanently deleted from the repository.

Trac ticket: https://core.trac.wordpress.org/ticket/65380

## Use of AI Tools
N/A

[desrosj]

In 62445:

Build/Test Tools: Remove unused reusable workflow.

The reusable-performance.yml workflow is no longer used in any branch, so it can be safely removed.

Props mukesh27.
Fixes #65380.