lib: Don't apply double spacing fix globally to all dropdowns

So that the SimpleSelect template looks like the deprecated Select
that it is replacing, and like the FormSelect that we are also using.

Instead, fix the dropdown margings in the Shell masthead, to keep them
as they were.

pkg/lib/patternfly/patternfly-5-overrides.scss

@@ -75,12 +75,6 @@ select.pf-v5-c-form-control {
   margin: 0;
 }
 
-// Fix the double-spacing issue for other dropdowns
-// https://github.com/patternfly/patternfly/issues/6632
-.pf-v5-c-menu-toggle__controls:last-child > .pf-v5-c-menu-toggle__toggle-icon {
-  margin-inline-end: 0;
-}
-
 // Fix menus cropping contents
 // https://github.com/patternfly/patternfly/issues/6565
 .pf-v5-c-menu__item-text {

pkg/shell/shell.scss

@@ -264,6 +264,12 @@ $desktop: $phone + 1px;
   --pf-v5-global--Color--100: var(--pf-v5-global--Color--dark-100);
 }
 
+// The normal margin to the right of the select dropdown makes them look uncentered in the masthead.
+// https://github.com/patternfly/patternfly/issues/6632
+.pf-v5-c-masthead .pf-v5-c-menu-toggle__controls:last-child > .pf-v5-c-menu-toggle__toggle-icon {
+  margin-inline-end: 0;
+}
+
 // Customize the about screen with the Cockpit logo
 .pf-v5-c-about-modal-box {
   --pf-v5-c-about-modal-box--BackgroundImage: linear-gradient(#000d, #000d), url(../shell/images/cockpit-icon.svg);