Remove some superfluous variables

[sit23]

Description

There are some variables in the model code that seem to be completely unused. For example, the variable pstd in constants.F90 is the reference pressure in dynes/cm2, is a namelist parameter, and does not appear to be used anywhere in the code. It would be good if this variable could be removed entirely to avoid confusion. This issue is then more of a reminder for us to look at this, and to try removing pstd to check if it is indeed superfluous. There may well be several other superfluous variables like this, so it would be good to check for others.