Fix problems reported by staticcheck

`staticcheck` is a golang code analysis tool. https://staticcheck.io/

This commit fixes a lot of problems found in our code. Common problems are:
- unnecessary use of fmt.Sprintf
- duplicated imports with different names
- unnecessary check that a key exists before a delete call

There are still a lot of reported problems in the test files but I have
not looked at those.

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

libpod/options.go

@@ -910,7 +910,7 @@ func WithUserNSFrom(nsCtr *Container) CtrCreateOption {
 		ctr.config.UserNsCtr = nsCtr.ID()
 		ctr.config.IDMappings = nsCtr.config.IDMappings
 
-		g := generate.NewFromSpec(ctr.config.Spec)
+		g := generate.Generator{Config: ctr.config.Spec}
 
 		g.ClearLinuxUIDMappings()
 		for _, uidmap := range nsCtr.config.IDMappings.UIDMap {