specs-go/config: Drop "this field is platform dependent" (again)

We dropped these in 4774080 (specs-go/config: Drop "this field is
platform dependent", 2016-09-14, opencontainers#568) but f9e48e0 (Windows: User
struct changes, 2016-09-14, opencontainers#565) was developed in parallel and
brought in a new one.

Signed-off-by: W. Trevor King <wking@tremily.us>

specs-go/config.go

@@ -61,7 +61,7 @@ type User struct {
 	GID uint32 `json:"gid" platform:"linux,solaris"`
 	// AdditionalGids are additional group ids set for the container's process.
 	AdditionalGids []uint32 `json:"additionalGids,omitempty" platform:"linux,solaris"`
-	// Username is the user name. (this field is platform dependent)
+	// Username is the user name.
 	Username string `json:"username,omitempty" platform:"windows"`
 }