stdenv/stripHash: print to stdout, not to variable

`stripHash` documentation states that it prints out the stripped name to
the stdout, but the function stored the value in `strippedName`
instead.

Basically all usages did something like
`$(stripHash $foo | echo $strippedName)` which is just braindamaged.
Fixed the implementation and all invocations.

pkgs/data/sgml+xml/schemas/xml-dtd/docbook-ebnf/builder.sh

@@ -3,5 +3,4 @@ source $stdenv/setup
 mkdir -p $out/xml/dtd/docbook-ebnf
 cd $out/xml/dtd/docbook-ebnf
 cp -p $dtd dbebnf.dtd
-stripHash $catalog
-cp -p $catalog $strippedName
+cp -p $catalog $(stripHash $catalog)