Remove SHELLFU_PATH from default setup
    
    SHELLFU_PATH brings more harm than good; developers are encouraged to
    put their private modules to regular shellfu path (in most cases
    /usr/share/shellfu/include-*) and use SHELLFU_PATH only for testing.
    It's a bit simpler (gets rid of one whole token) and has the advantage
    of exposing the library to sfdoc.