Remove TODO comments about spacing variables (#10695)

The variables on _spacing.pcss have been deprecated by https://github.com/matrix-org/matrix-react-sdk/pull/10686

res/css/views/settings/tabs/_SettingsSection.pcss

@@ -16,7 +16,7 @@ limitations under the License.
 
 .mx_SettingsSection {
     --SettingsTab_section-margin-bottom-preferences-labs: 30px;
-    --SettingsTab_heading_nth_child-margin-top: 30px; /* TODO: Use a spacing variable */
+    --SettingsTab_heading_nth_child-margin-top: 30px;
     --SettingsTab_tooltip-max-width: 120px; /* So it fits in the space provided by the page */
 
     color: $primary-content;

res/css/views/settings/tabs/_SettingsTab.pcss

@@ -50,9 +50,9 @@ limitations under the License.
     color: $secondary-content;
     font-size: $font-14px;
     display: block;
-    margin-top: 10px; /* TODO: Use a spacing variable */
+    margin-top: 10px;
     margin-inline-end: 80px; /* Align with the rest of the view */
-    margin-bottom: 10px; /* TODO: Use a spacing variable */
+    margin-bottom: 10px;
     margin-inline-start: 0;
 }
 
@@ -63,7 +63,7 @@ limitations under the License.
 
     .mx_SettingsFlag {
         margin-inline-end: $end-gutter;
-        margin-bottom: 10px; /* TODO: Use a spacing variable */
+        margin-bottom: 10px;
 
         .mx_SettingsFlag_label {
             vertical-align: middle;