Remove no longer needed diagram cache file excluding/deleting

The current version of the diagram extension now generates these files
only under the build directory, so they are automatically excluded from
Git and deleted on clean.

.gitignore

@@ -17,8 +17,5 @@ gradle-app.setting
 
 
 combined.bib
-*.cache
-diag-*.png.cache
-diag-*.png
 tmp
 src/docs/asciidoc/*.adoc

build.gradle

@@ -124,9 +124,6 @@ asciidoctor {
 }
 
 clean {
-    delete(fileTree('src/docs/asciidoc/images') {
-        include '**/diag-*'
-    })
     delete(fileTree('src/docs/asciidoc') {
         include '*.adoc'
     })