remove unnecessary shp2pgsql import

Looks like this was leftover from a copy/paste. An import exists for
ne_110m_ocean-merc.shp file, and that's the name of the file that exists
in the water-areas-naturalearth.zip file.
refactor updates into function calls

Extract the logic for calculating the values for new columns into
functions to prepare for triggers.
rename function parameter
spacing
create trigger functions and triggers