parser: do not try to continue with unsafe on foreign fns

The changed line makes it look like `unsafe` is allowed, but the
first statement of `parse_item_foreign_fn` is:

`self.expect_keyword(keywords::Fn)?;`

So we get the strange "expected one of `fn`, `pub`, `static`, or
`unsafe`, found `unsafe`".

Fixes: #27361

src/libsyntax/parse/parser.rs

@@ -6025,7 +6025,7 @@ impl<'a> Parser<'a> {
             // FOREIGN STATIC ITEM
             return Ok(Some(self.parse_item_foreign_static(visibility, lo, attrs)?));
         }
-        if self.check_keyword(keywords::Fn) || self.check_keyword(keywords::Unsafe) {
+        if self.check_keyword(keywords::Fn) {
             // FOREIGN FUNCTION ITEM
             return Ok(Some(self.parse_item_foreign_fn(visibility, lo, attrs)?));
         }