[SYCL][NFC] Drop legacy dockerfiles (#17785)

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

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

Author: AlexeySachkov