[Hygiene] Remove include_view_image_tool config (#5976)

There's still some debate about whether we want to expose
`tools.view_image` or `feature.view_image` so those are left unchanged
for now, but this old `include_view_image_tool` config is good-to-go.
Also updated the doc to reflect that `view_image` tool is now by default
true.

codex-rs/app-server/src/codex_message_processor.rs

@@ -1777,7 +1777,6 @@ async fn derive_config_from_params(
         developer_instructions,
         compact_prompt,
         include_apply_patch_tool,
-        include_view_image_tool: None,
         show_raw_agent_reasoning: None,
         tools_web_search_request: None,
         experimental_sandbox_command_assessment: None,