Version Control With Subversion Source

doing so, bump the claimed DocBook version to 4.5.

* en/README
Note that the book is written in DocBook 4.5 now.

* en/book/book.xml
Point to the DocBook 4.5 DTD.

* tools/Makefile.base-po
Remove pre-processing of source files to refer to DocBook 4.5 -- the
source already so refers.

* tools/dtd
Remove these local copies of the Docbook DTDs.