cli/command/stack/swarm: waitOnServices remove redundant check for multi-error

[thaJeztah]

This check was redundant, because errors.Join already checks if the
list of errors is either empty, or only contains nil errors, as can
be seen in this example;

package main

import (
    "errors"
    "testing"
)

func TestMultiErr(t *testing.T) {
    var errs []error
    if err := errors.Join(errs...); err != nil {
        t.Fatal(err)
    }

    errs = append(errs, nil, nil, nil)
    t.Logf("errs contains %d elements", len(errs))
    if err := errors.Join(errs...); err != nil {
        t.Fatal(err)
    }

    errs = append(errs, errors.New("with an error"))
    if err := errors.Join(errs...); err == nil {
        t.Fatal("expected an error")
    }
}

- Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 59.42%. Comparing base (d68c4d8) to head (dea59ea).
Report is 2 commits behind head on master.

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5780   +/-   ##
=======================================
  Coverage   59.42%   59.42%           
=======================================
  Files         347      347           
  Lines       29398    29395    -3     
=======================================
  Hits        17469    17469           
+ Misses      10957    10954    -3     
  Partials      972      972