use pdfreader instead of crereader to handle zip files

Generally zip files are zipped pictures which should be
better handled in pdfreader.

frontend/document/credocument.lua

@@ -396,7 +396,6 @@ function CreDocument:register(registry)
     registry:addProvider("pdb", "application/pdb", self)
     registry:addProvider("doc", "application/doc", self)
     registry:addProvider("tcr", "application/tcr", self)
-    registry:addProvider("zip", "application/zip", self)
 end
 
 return CreDocument

frontend/document/pdfdocument.lua

@@ -224,6 +224,7 @@ end
 function PdfDocument:register(registry)
     registry:addProvider("pdf", "application/pdf", self)
     registry:addProvider("cbz", "application/cbz", self)
+    registry:addProvider("zip", "application/zip", self)
     registry:addProvider("xps", "application/xps", self)
 end