[SYCL]: Remove per-allocation Level Zero runtime checks (#23399)

[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