[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/exec/src/lib.rs

@@ -177,7 +177,6 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         developer_instructions: None,
         compact_prompt: None,
         include_apply_patch_tool: None,
-        include_view_image_tool: None,
         show_raw_agent_reasoning: oss.then_some(true),
         tools_web_search_request: None,
         experimental_sandbox_command_assessment: None,