Remove ecosystem_ci flag from Ruff CLI (#12596)

## Summary

@zanieb noticed while we were discussing #12595 that this flag is now
unnecessary, so remove it and the flags which reference it.

## Test Plan

Question for maintainers: is there a test to add *or* remove here? (I’ve
opened this as a draft PR with that in view!)

scripts/check_ecosystem.py

@@ -191,7 +191,7 @@ async def check(
     if exclude:
         ruff_args.extend(["--exclude", exclude])
     if show_fixes:
-        ruff_args.extend(["--show-fixes", "--ecosystem-ci"])
+        ruff_args.extend(["--show-fixes"])
 
     start = time.time()
     proc = await create_subprocess_exec(