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.

mkit.ini

     __SATURNIN_CONFIG_LOCAL__ = [roots:etc]
     __SATURNIN_LIBEXEC__      = [roots:libexec]
     __SATURNIN_SHARE__        = [roots:share]
-    __SATURNIN_SHELLFU_DIR__  = [roots:share]/shellfu
 
 [modes]
     bin     = 755

src/app.skel

     SATURNIN_META_HELP=${SATURNIN_META_HELP:-"+__SATURNIN_SHARE__/help"}
     SATURNIN_LIBEXEC=${SATURNIN_LIBEXEC:-"__SATURNIN_LIBEXEC__"}
     SATURNIN_LIBEXEC_PREFIX="__MKIT_PROJ_PKGNAME__-"
-    SHELLFU_PATH=${SHELLFU_PATH:-"__SATURNIN_SHELLFU_DIR__"}
     PRETTY_USAGE="subcommand"
     PRETTY_DEBUG_EXCLUDE="$PRETTY_DEBUG_EXCLUDE,inigrep,saturnin"
 }