[SYCL] remove experimental online_compiler extension (#16776)

Minus support for "CM", all the functionality of the old experimental
online_compiler extension is being provided by the new (also
experimental) kernel_compiler. Only better.

Besides always being merely experimental, the online_compiler has been
marked as deprecated for over a year, it has become burdensome to
continue to support, and it sometimes confuses users. The decision has
been made to remove it, without waiting for an ABI breaking window.

Author: cperkinsintel