[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