Don't fire variable set event before entering a for-loop

Since #4376, for-loops would set the loop variable outside, so it
stays valid.

They did this by doing the equivalent of

```fish
set -l foo $foo
for foo in 1 2 3
```

And that first imaginary `set -l` would also fire a set-event.

Since there's no use for it and the variable isn't actually set, we
remove it.

Fixes #8384.

tests/checks/for.fish

@@ -32,3 +32,14 @@ begin
 end
 echo $k
 # CHECK: global
+
+function foo --on-variable foo
+    echo foo set
+end
+
+for foo in 1 2 3
+    true
+end
+# CHECK: foo set
+# CHECK: foo set
+# CHECK: foo set