Drop redundant guard cases

The upper index bound for a slice is cap(args) we can safely retun appended slices

```
~>  go test -count=1 ./plugin/... -v
=== RUN   TestPluginServerRandom
--- PASS: TestPluginServerRandom (0.00s)
=== RUN   TestSet
--- PASS: TestSet (0.00s)
=== RUN   TestSetProtobufArgParsing
=== RUN   TestSetProtobufArgParsing/no_--protobuf_argument_provided
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_first_argument
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_last_argument
=== RUN   TestSetProtobufArgParsing/providing_--protobuf_as_middle_argument
--- PASS: TestSetProtobufArgParsing (0.00s)
    --- PASS: TestSetProtobufArgParsing/no_--protobuf_argument_provided (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_first_argument (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_last_argument (0.00s)
    --- PASS: TestSetProtobufArgParsing/providing_--protobuf_as_middle_argument (0.00s)
PASS
ok      github.com/hashicorp/packer-plugin-sdk/plugin   0.244s

```

plugin/set.go

@@ -133,15 +133,6 @@ func (i *Set) parseProtobufFlag(args ...string) []string {
 	}
 
 	i.useProto = true
-
-	if protobufPos == 0 {
-		return args[1:]
-	}
-
-	if protobufPos == len(args)-1 {
-		return args[:len(args)-1]
-	}
-
 	return append(args[:protobufPos], args[protobufPos+1:]...)
 }