chore: implement delete organization member (#13589)

Side effects of removing an organization member will orphan their
user resources. These side effects are not addressed here

coderd/database/queries/organizationmembers.sql

@@ -36,6 +36,15 @@ INSERT INTO
 VALUES
 	($1, $2, $3, $4, $5) RETURNING *;
 
+-- name: DeleteOrganizationMember :exec
+DELETE
+	FROM
+		organization_members
+	WHERE
+		organization_id = @organization_id AND
+		user_id = @user_id
+;
+
 
 -- name: GetOrganizationIDsByMemberIDs :many
 SELECT