Clean up the rewritten deco.c

In commit a7902f279a57 "Rewrite of the deco code" Robert kept the old code
that he replaced around; I removed all that as it's in the git history if
we ever need to look at it again but doesn't really help us in the file as
it is.

I also removed constants, variables and config parameters that aren't used
in the new implementation and did some coding style / formatting changes
to make deco.c more consistent with the rest of Subsurface. I also updated
the comments at the top of the file to reflect reality.

I did one change that actually affects the code. In the explanation of his
changes Robert said that gf_low_pressure_this_dive is initialized to the
exquivalent of 20m, yet his code added the surface pressure twice. I
decided to change the default config value from 3 (bar) to 2 so that this
indeed reflects (about) 20m (as in the code below surface_pressure is
added to this value).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>