Remove redundant duplicate pressure samples

At least the Suunto pressure transmitter seems to be pretty
"quantisized", and it will send identical samples for a while until the
pressure changes enough.  Then subsurface gives this silly flat line
with a sudden jump downwards, which *could* be you suddenly taking a
deep breath after holding it for a while, but almost certainly it's a
sensor issue.

So just remove successive identical pressure readings.  They aren't
interesting, and subsurface will actually do a good job of interpolating
it according to SAC rate instead.  And they just make the XML look
worse.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>