[SYCL][NFC] Drop legacy dockerfiles

Those dockerfiles were previosly renamed on `sycl` branch in
intel#16255 and intel#16680.

When we synced `.github` and `devops` changes from `sycl` to
`sycl-rel-6_1_0` in intel#17612, we copied over new versions, but
did not delete the old ones and this PR does exactly that.

Author: AlexeySachkov