Cleanup GGML_OPENVINO_* ENV variable handling

[ravi9]

All GGML_OPENVINO_* environment variables should be read exactly once, ideally during backend registration or inside ggml_openvino_device_config::init() and stored as fields in the ggml_openvino_device_config struct. The rest of the code should access them through the existing ggml_openvino_get_device_config() accessor rather than calling getenv directly. This would:

• Eliminate repeated getenv calls inside ov_graph_compute_dynamic() and ov_graph_compute_static(), which are invoked on every inference step.
• Remove the need for the read-and-unset hack for GGML_OPENVINO_PRINT_CGRAPH_TENSOR_ADDRESS.
• Consolidate all configurations in one place, making it easier to document, maintain, and extend.

[mostafafaheem]

.take

[ravi9]

Updated with: 835121d

[ravi9]

ggml-org#24503