Redundant namespace declarations

A namespace prefix declared on the xsl:stylesheet element but never used by any element name, attribute name, or qualified name inside an attribute value is dead weight that misleads the reader and should be removed.

Two attributes use a prefix without qualifying anything with it, and a prefix either of them names is in use: exclude-result-prefixes keeps the namespace out of the result tree, and extension-element-prefixes marks it as holding extension instructions. Both hold bare prefixes, separated by whitespace, and exclude-result-prefixes="#all" names every prefix in scope at once. Removing a declaration one of them names leaves the reference bound to nothing, and a conformant processor refuses to compile the stylesheet.

Incorrect:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:foo="urn:foo" version="2.0">
  <!-- the foo prefix is never used anywhere -->
</xsl:stylesheet>

Correct:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
  <!-- only namespaces that are actually used are declared -->
</xsl:stylesheet>

A prefix whose only mention is a prefix list is used, and its declaration stays:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:foo="urn:foo"
  exclude-result-prefixes="foo" version="2.0">
  <!-- foo is declared so that exclude-result-prefixes can name it -->
</xsl:stylesheet>