Eliminates pure-print-callbacks list (issue 7300082)

[dak]

https://codereview.appspot.com/7300082/diff/1/scm/define-grobs.scm
File scm/define-grobs.scm (right):

https://codereview.appspot.com/7300082/diff/1/scm/define-grobs.scm#newcode2570
scm/define-grobs.scm:2570: (Y-extent . ,pure-safe-stencil-height)
Are there grobs that don't have this definition of Y-extent?  If it is
the same everywhere, do we even need an entry for it?

https://codereview.appspot.com/7300082/diff/1/scm/output-lib.scm
File scm/output-lib.scm (right):

https://codereview.appspot.com/7300082/diff/1/scm/output-lib.scm#newcode61
scm/output-lib.scm:61: (define-public pure-safe-stencil-height
Perhaps add any information about the name?  Its name claims to produce
a pure value, but it actually outputs callbacks both for pure and
unpure.  What makes it "safe"?

https://codereview.appspot.com/7300082/