c: remove inline 'backing' type tag for enums

It was meant as a micro-optimization but actually keeping it in sync was
annoying. This for example was broken:

```
extern enum foo { ... } g_foo;
enum foo g_foo;
```

1 files changed, 0 insertions, 1 deletions

diff --git a/src/c.c b/src/c.c
index d6879ad..6446ab5 100644
--- a/src/c.c
+++ b/src/c.c
@@ -2198,7 +2198,6 @@ buildenum(CComp *cm, internstr name, const Span *span, int id)
       ty = completetype(name, id, &td);
    else
       ty = mktagtype(name, &td);
-   ty.backing = td.backing;
    return ty;
 }