openjdk10: minor cleanups

* with only one source bundle (per JEP-296), we can use src instead of
  srcs, and avoid the need to cd in prePatch

* fetch sources from jdk10u instead of jdk10, to make it easier to
  grab updates when they start coming.

* removed commented-out code that became irrelevant in the 8 -> 9
  transition (*.pf files, infinality font rendering)

* create jdk10, jre10, and jre10_headless attributes in
  all-packages.nix