[#388] *: Remove nil check from setters

I knew one day `sed` would save me an hour of manual work:
```
sed -i -n -e '
s/) Set/) Set/
p
t setter
b end
:setter
    n
    s/nil/nil/
    t hasif
    p
    b end
    :hasif
        n
        :loop
        p
        n
        s/}/}/
        t end
        b loop
        :end
' $@
goimports -w $@
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

container/grpc/types.go

@@ -7,56 +7,40 @@ import (
 
 // SetKey sets key to the container attribute.
 func (m *Container_Attribute) SetKey(v string) {
-	if m != nil {
-		m.Key = v
-	}
+	m.Key = v
 }
 
 // SetValue sets value of the container attribute.
 func (m *Container_Attribute) SetValue(v string) {
-	if m != nil {
-		m.Value = v
-	}
+	m.Value = v
 }
 
 // SetOwnerId sets identifier of the container owner,
 func (m *Container) SetOwnerId(v *refs.OwnerID) {
-	if m != nil {
-		m.OwnerId = v
-	}
+	m.OwnerId = v
 }
 
 // SetNonce sets nonce of the container structure.
 func (m *Container) SetNonce(v []byte) {
-	if m != nil {
-		m.Nonce = v
-	}
+	m.Nonce = v
 }
 
 // SetBasicAcl sets basic ACL of the container.
 func (m *Container) SetBasicAcl(v uint32) {
-	if m != nil {
-		m.BasicAcl = v
-	}
+	m.BasicAcl = v
 }
 
 // SetAttributes sets list of the container attributes.
 func (m *Container) SetAttributes(v []*Container_Attribute) {
-	if m != nil {
-		m.Attributes = v
-	}
+	m.Attributes = v
 }
 
 // SetPlacementPolicy sets placement policy of the container.
 func (m *Container) SetPlacementPolicy(v *netmap.PlacementPolicy) {
-	if m != nil {
-		m.PlacementPolicy = v
-	}
+	m.PlacementPolicy = v
 }
 
 // SetVersion sets version of the container.
 func (m *Container) SetVersion(v *refs.Version) {
-	if m != nil {
-		m.Version = v
-	}
+	m.Version = v
 }