fix(Instance/Volume): size_in_db should not be set when importing volume

[jeansebastienh]

When creating a volume from a snapshot or with a copy: the attribute "size_in_db" should not be read as it's conflicting with this feature

The test was failing:

resource_instance_volume_test.go:89: Step 1/1 error: After applying this test step, the plan was not empty.                                                                                                                               
        stdout:                                                                                                        
                                                                                                                                                                                                                                              
                                                                                                                                                                                                                                              
        An execution plan has been generated and is shown below.
        Resource actions are indicated with the following symbols:
          ~ update in-place
                                                         
        Terraform will perform the following actions:    
                                                          
        # scaleway_instance_volume.test2 will be updated in-place
          ~ resource "scaleway_instance_volume" "test2" {
                id              = "fr-par-1/ac5b205e-a19f-48a5-a7a0-2161b3736cf3"                                      
                name            = "tf-vol-wizardly-kilby"
              - size_in_gb      = 20 -> null                                                                           
                # (5 unchanged attributes hidden)
            }                              
                                                  
        Plan: 0 to add, 1 to change, 0 to destroy.

Now

--- PASS: TestAccScalewayInstanceVolume_FromVolume (1.91s)                                                             
PASS                                                                                                                   
ok      github.com/scaleway/terraform-provider-scaleway/scaleway        1.922s