Remove rules for building concept/implementation documentation

Instead, these documents are captured in 'compiled' form (pdf/ps/txt).

This means we no longer need dependency on groff

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=74793aa03131736912f64b460541e842fc9ba8fc

Modified Files
--------------
RELEASE                                            |    6 +-
doc/Makefile                                       |    2 +-
doc/concept/Makefile                               |  131 -
doc/concept/Slon.svg                               | 2478 ---------
doc/concept/Slon_900x720.jpg                       |  Bin 40224 -> 0 bytes
doc/concept/Slony-I-concept.nr                     |  934 ----
doc/concept/figure-1.pic                           |   35 -
doc/concept/figure-2.pic                           |   50 -
doc/concept/figure-3.pic                           |   40 -
doc/concept/figure-4.pic                           |   38 -
doc/historical/concept/Slon.eps                    | 3695 +++++++++++++
doc/historical/concept/Slon.svg                    | 2478 +++++++++
doc/historical/concept/Slon_900x720.jpg            |  Bin 0 -> 40224 bytes
doc/historical/concept/Slony-I-concept.pdf         |  Bin 0 -> 94721 bytes
doc/historical/concept/Slony-I-concept.ps          | 5438 ++++++++++++++++++++
doc/historical/concept/Slony-I-concept.txt         | 1254 +++++
doc/historical/implementation/Slon.eps             | 3695 +++++++++++++
doc/historical/implementation/Slon_900x720.jpg     |  Bin 0 -> 40224 bytes
.../implementation/Slony-I-implementation.pdf      |  Bin 0 -> 55203 bytes
.../implementation/Slony-I-implementation.ps       | 4471 ++++++++++++++++
.../implementation/Slony-I-implementation.txt      |  528 ++
doc/implementation/Makefile                        |  128 -
doc/implementation/Slon_900x720.jpg                |  Bin 40224 -> 0 bytes
doc/implementation/Slony-I-implementation.nr       |  315 --
doc/implementation/figure-1.pic                    |  159 -
doc/implementation/figure-2.pic                    |   72 -
26 files changed, 21565 insertions(+), 4382 deletions(-)