`runsc gofer`: Remove `/proc` unmounter and unmount in-process instead

Saves about 3ms on the gofer hot path.

Benchmarks:

```
                 │     base     │                  new                  │
                 │    sec/op    │    sec/op     vs base                 │
  RunscDo           75.78m ± 3%   73.06m ±  2%   -3.59% (p=0.012 n=200)
  GoferInit        10.403m ± 5%   7.364m ± 13%  -29.21% (p=0.000 n=25)
```