dspace.cfg: remove old webui.browse.thumbnail.show property

The webui.browse.thumbnail.show property is no longer used as of
DSpace 7. Also remove subsequent references to adding thumbnails
to item view columns since these are from legacy DSpace.

dspace/config/dspace.cfg

@@ -1460,17 +1460,14 @@ log.report.dir = ${dspace.dir}/log
 # fields at least the date and title fields as specified by the
 # webui.browse.index.* configuration options below.
 #
-# If you have enabled thumbnails (webui.browse.thumbnail.show), you must also
-# include a 'thumbnail' entry in your columns - this is where the thumbnail will be displayed
-#
 # If you want to mark each item include a 'mark_[value]' (without the brackets - replace the word 'value' with anything that
 # has a meaning for your mark) entry in your columns - this is where the icon will be displayed.
 # Do not forget to add a Spring bean with id = "org.dspace.app.itemmarking.ItemMarkingExtractor.[value]"
 # in file 'config/spring/api/item-marking.xml'. This bean is responsible for drawing the appropriate mark for each item.
 # You can add more than one 'mark_[value]' options (with different value) in case you need to mark items more than one time for
 # different purposes. Remember to add the respective beans in file 'config/spring/api/item-marking.xml'.
 #
-# webui.itemlist.columns = thumbnail, dc.date.issued(date), dc.title, dc.contributor.*
+# webui.itemlist.columns = dc.date.issued(date), dc.title, dc.contributor.*
 #
 # Additionally, you can override the DC fields used on the listing page for
 # a given browse index and/or sort option. As a sort option or index may be defined
@@ -1489,14 +1486,6 @@ log.report.dir = ${dspace.dir}/log
 # In the last case, a sort option name will always take precedence over a browse
 # index name. Note also, that for any additional columns you list, you will need to
 # ensure there is an itemlist.<field name> entry in the messages file.
-#
-# The following example would display the date of accession in place of the issue date
-# whenever the dateaccessioned browse index or sort option is selected.
-#
-# Just like webui.itemlist.columns, you will need to include a 'thumbnail' entry to display
-# and thumbnails in the item list
-#
-# webui.itemlist.dateaccessioned.columns = thumbnail, dc.date.accessioned(date), dc.title, dc.contributor.*
 
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