[SYCL] Remove fusion scheduler integration

[sommerlukas]

Remove the integration of kernel fusion from the SYCL runtime scheduler. This makes the implementation of the scheduler less complex and leaner.

As a consequence, the experimental kernel fusion feature is no longer supported, the extension proposal status has been updated.

Some stubs of the kernel fusion must remain for now to avoid ABI-break, removing those stubs in the next ABI-breaking window is tracked through #15184.