confirmDelete (and confirmDeleteAll) already say why a build cannot be deleted...

confirmDelete (and confirmDeleteAll) already say why a build cannot be deleted (and prevent you from proceeding), so no need to hide the Delete link in this case.

core/src/main/resources/hudson/matrix/MatrixBuild/delete.jelly

@@ -27,7 +27,7 @@ THE SOFTWARE.
 -->
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
-  <j:if test="${!it.keepLog and !it.building}">
+  <j:if test="${!it.building}">
     <l:task icon="images/24x24/edit-delete.png" href="${buildUrl.baseUrl}/confirmDelete" title="${%Delete Build}" permission="${it.DELETE}" />
   	<l:task icon="images/24x24/edit-delete.png" href="${buildUrl.baseUrl}/confirmDeleteAll" title="${%Delete this build and all configurations in this build}" permission="${it.DELETE}" />
   </j:if>

core/src/main/resources/hudson/model/Run/delete.jelly

@@ -27,7 +27,7 @@ THE SOFTWARE.
 -->
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
-  <j:if test="${!it.keepLog and !it.building}">
+  <j:if test="${!it.building}">
     <l:task icon="images/24x24/edit-delete.png" href="${buildUrl.baseUrl}/confirmDelete" title="${%Delete this build}" permission="${it.DELETE}" />
   </j:if>
 </j:jelly>
\ No newline at end of file