Revert "Add text/vnd.tiddlywiki to system tiddlers within plugins that should be wikified (c.f. #2883)"

This reverts commit 7436fc7374.

plugins/tiddlywiki/qrcode/doc/examples.tid

@@ -1,4 +1,3 @@
 title: $:/plugins/tiddlywiki/qrcode/examples
-type: text/vnd.tiddlywiki
 
 <<tabs "[all[shadows+tiddlers]tag[$:/tags/MakeQR]!has[draft.of]]" "$:/plugins/tiddlywiki/qrcode/MakeGenericQR">>

plugins/tiddlywiki/qrcode/doc/readme.tid

@@ -1,5 +1,4 @@
 title: $:/plugins/tiddlywiki/qrcode/readme
-type: text/vnd.tiddlywiki
 
 The QR code plugin provides a macro that enables any text to be rendered as a [[QR code|https://en.wikipedia.org/wiki/QR_code]]. QR codes are a type of 2-dimensional bar code that encodes arbitrary data: text, numbers, links. QR code readers are available or built-in for smartphones, making them a convenient means to transfer information between devices
 

plugins/tiddlywiki/qrcode/doc/usage.tid

@@ -1,5 +1,4 @@
 title: $:/plugins/tiddlywiki/qrcode/usage
-type: text/vnd.tiddlywiki
 
 ! `makeqr` Macro