[SYCL]: Remove per-allocation Level Zero runtime checks (#23399)
* [SYCL] Centralize Level Zero detection in ggml_sycl_init
* use the same wording
* get back the warning
* [SYCL] Remove per-allocation getenv() for GGML_SYCL_ENABLE_LEVEL_ZERO
* bring back the comment
* move it up to make sure devices call the shots
* move the env detection early
* replace g_ggml_sycl_enable_level_zero with a direct call to .ext_oneapi_level_zero
* update the comment
* switch back to g_ggml_sycl_enable_level_zero with a sentinel
* remove the check
* Reduce the diff
* reword, move lower
* move things aroudn
* remove forward declaration if favor of a full replace
* pre-cache results of zeDeviceGetProperties
* put ggml_sycl_get_env back
* replace get_sycl_env with ggml_sycl_get_env
* add whitespace back
* Apply suggestion from @sanmai