Codechange: Use existing functions in GetMonthlyInfrastructureCosts.

[J0anJosep]

Motivation / Problem

Not sure about this one, but there is some duplicate/similar code related with infrastructure that can be replaced with already existing functions.

Description

Replace some code with calls to already existing functions.

Limitations

For road infrastructure, it will call two functions that can't be inlined.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[TrueBrain]

Makes sense.