remove path length parameter from field_manager

[rem1776]

Is your feature request related to a problem? Please describe.
There is a a variable for file/path name length fm_path_name_len set in the field manager. This should be removed when possible so we can use consistent lengths across the code instead of defining different lengths in different areas.

Describe the solution you'd like
Once a consistent path length is added to FMS, we should remove usage of the field_manager-specific variable (used in land_lad2 and ocean_BGC), and then remove it from field_manager.

Describe alternatives you've considered
Keeping the variable as is, it will now be set to a parameter set in platform_mod.

Additional context
came up while working on #1518