Remove reference to spurious "foo" variable

Prevents the following byte-compilation warning:

```
skewer-mode.el:562:9:Warning: assignment to free variable `foo'
```

Note that a number of other warnings remain:

```
skewer-mode.el:126:1:Warning: cl package required at runtime
skewer-mode.el:182:1:Warning: Unused lexical argument `error-case'
skewer-mode.el:254:1:Warning: Unused lexical argument `args'
skewer-mode.el:254:1:Warning: Unused lexical argument `query'
skewer-mode.el:254:1:Warning: Unused lexical argument `path'
skewer-mode.el:257:1:Warning: Unused lexical variable `type'
skewer-mode.el:257:1:Warning: Unused lexical argument `args'
skewer-mode.el:257:1:Warning: Unused lexical argument `query'
skewer-mode.el:257:1:Warning: Unused lexical argument `path'
```