Remove bundled zlib

* Link zlib with '-lz' on Windows

* inst/COPYING: Remove zlib license from file

* scripts/build_Makevars.r: Update to build Makevars.win with -lz and
  remove OBJECTS.zlib

* src/Makevars.win: Update

* src/zlib: Delete files and folder

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update .Rbuildignore

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>