Setup and use for macOS of GRASS_HTML_BROWSER in init and g.manual

[wenzeslaus]

Setup of GRASS_HTML_BROWSER in lib/init/grass.py does not make much sense considering the possible branches in the code. Furthermore, g.manual relies on the webbrowser package, making the whole platform-specific setup of GRASS_HTML_BROWSER less useful. g.manual is the only place where GRASS_HTML_BROWSER is used.