[SecuritySolution] Remove empty row (#129909)

* fix: remove horizontal rule

The horizontal rule in this component in combination with the bottom border of the above table made it look like there is an empty row in the table. The divider is now replaced by a spacer to visually separate the Investigation guide and the highlighted fields.

* chore: remove unused import

x-pack/plugins/security_solution/public/common/components/event_details/investigation_guide_view.tsx

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { EuiSpacer, EuiHorizontalRule, EuiTitle, EuiText } from '@elastic/eui';
+import { EuiSpacer, EuiTitle, EuiText } from '@elastic/eui';
 import { ALERT_RULE_UUID } from '@kbn/rule-data-utils';
 
 import React, { useMemo } from 'react';
@@ -39,7 +39,7 @@ const InvestigationGuideViewComponent: React.FC<{
 
   return (
     <>
-      <EuiHorizontalRule />
+      <EuiSpacer size="l" />
       <EuiTitle size="xxxs" data-test-subj="summary-view-guide">
         <h5>{i18n.INVESTIGATION_GUIDE}</h5>
       </EuiTitle>