libcephfs_proxy: remove redundant unsigned >= 0 comparison

libcephfs_proxy: remove redundant unsigned >= 0 comparison

Fix a clang warning in proxy_async.c where an unsigned value was being
unnecessarily compared against 0:
```
/home/kefu/dev/ceph/src/libcephfs_proxy/proxy_async.c:29:12: warning: result of comparison of unsigned expression >= 0 is always true [-Wtautological-unsigned-zero-compare]
   29 |         if ((size >= 0) && !info->write) {
      |              ~~~~ ^  ~
1 warning generated.
```

Since unsigned values are always >= 0 by definition, remove this
tautological check to resolve the "-Wtautological-unsigned-zero-compare"
warning.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>