Remove unused XML Signature schema and dtd files from source

The following files are unused and not included in any of the JDK or JRE binaries:

jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/xenc-schema.xsd
jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/xenc-schema.rng
jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/xmldsig-core-schema.rng
jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/xmldsig-core-schema.xsd
jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/xmldsig-core-schema.dtd
jdk/src/share/classes/com/sun/org/apache/xml/internal/security/resource/schema/etsi.xsd

They should be removed.