Stop doing the "symlink to rev-labeled PDF" trick

Before now, building "foo.pdf" would instead build "foo-rREVNUM.pdf"
(where REVNUM is determined by 'get_revision') and symlink "foo.pdf"
to that.  Although this had its uses, it also would occasionally cause
problems, primarily accidentally copying the symlink to somewhere else
and ending up with a broken symlink at the destination.  After
discussing it yesterday, James and I decided to switch to the simpler
"just build foo.pdf" method, and that's what this commit does.