Fix removing existing users from groups

[winglot]

A fix to allow removing existing users from groups.

The added test case replicates an error reported by @tripolif98 in #79.

=== RUN   TestAccRedshiftGroup_RemoveExistingUser
    resource_redshift_group_test.go:145: Step 2/2 error: Error running apply: exit status 1

        Error: error reading info about user: %!s(<nil>)

          with redshift_group.group,
          on terraform_plugin_test.tf line 2, in resource "redshift_group" "group":
           2: resource "redshift_group" "group" {

--- FAIL: TestAccRedshiftGroup_RemoveExistingUser (13.19s)
FAIL
FAIL    github.com/brainly/terraform-provider-redshift/redshift 13.200s
FAIL

After fix to the switch statement is applied, the test passes.

=== RUN   TestAccRedshiftGroup_RemoveExistingUser
--- PASS: TestAccRedshiftGroup_RemoveExistingUser (16.52s)
PASS
ok      github.com/brainly/terraform-provider-redshift/redshift 16.522s