chore(header): remove excess bottom margin (#91753)

**Before & After:**
<img width="484" alt="Screenshot 2025-05-15 at 11 25 14 AM"
src="https://github.com/user-attachments/assets/c03630b0-857b-4533-896a-e717ee07d702"
/>
<img width="606" alt="Screenshot 2025-05-15 at 11 24 53 AM"
src="https://github.com/user-attachments/assets/66cc626c-2a30-4e67-a63d-b782c03f89c2"
/>

Author: doralchan

static/app/components/layouts/thirds.tsx

@@ -60,15 +60,8 @@ export const HeaderContent = styled('div')<{unified?: boolean}>`
   display: flex;
   flex-direction: column;
   justify-content: normal;
-  margin-bottom: ${space(1)};
   overflow: hidden;
   max-width: 100%;
-
-  ${p =>
-    p.unified &&
-    css`
-      margin-bottom: 0;
-    `}
 `;
 
 /**

static/app/views/explore/multiQueryMode/index.tsx

@@ -37,7 +37,7 @@ export default function MultiQueryMode() {
       renderDisabled={NoAccess}
     >
       <SentryDocumentTitle title={t('Compare Queries')} orgSlug={organization.slug}>
-        <Layout.Header>
+        <Layout.Header unified>
           <Layout.HeaderContent>
             <Breadcrumbs
               crumbs={[