maint: Remove unneeded input validation from internal fcns in private/ directories. * scripts/general/private/__isequal__.m, scripts/general/private/__splinen__.m, scripts/image/private/__imwrite__.m, scripts/image/private/ind2x.m, scripts/miscellaneous/private/__xzip__.m, scripts/miscellaneous/private/display_info_file.m, scripts/pkg/private/describe.m, scripts/pkg/private/get_forge_pkg.m, scripts/pkg/private/unload_packages.m, scripts/plot/private/__actual_axis_position__.m, scripts/plot/private/__add_datasource__.m, scripts/plot/private/__clabel__.m, scripts/plot/private/__errcomm__.m, scripts/plot/private/__errplot__.m, scripts/plot/private/__fltk_print__.m, scripts/plot/private/__gnuplot_get_var__.m, scripts/plot/private/__go_draw_axes__.m, scripts/plot/private/__go_draw_figure__.m, scripts/plot/private/__interp_cube__.m, scripts/plot/private/__line__.m, scripts/plot/private/__next_line_color__.m, scripts/plot/private/__next_line_style__.m, scripts/plot/private/__plt__.m, scripts/plot/private/__pltopt__.m, scripts/signal/private/rectangle_lw.m, scripts/signal/private/rectangle_sw.m, scripts/signal/private/triangle_lw.m, scripts/signal/private/triangle_sw.m, scripts/sparse/private/__sprand_impl__.m, scripts/statistics/models/private/logistic_regression_derivatives.m, scripts/statistics/models/private/logistic_regression_likelihood.m: Remove unneeded input validation from internal fcns in private/ directories.

--- a/scripts/miscellaneous/private/__xzip__.m
+++ b/scripts/miscellaneous/private/__xzip__.m
@@ -33,14 +33,6 @@
 function entries = __xzip__ (commandname, extension,
                              commandtemplate, files, outdir)
 
-  if (nargin != 4 && nargin != 5)
-    print_usage ();
-  endif
-
-  if (! ischar (extension) || length (extension) == 0)
-    error ("__xzip__: EXTENSION must be a string with finite length");
-  endif
-
   if (nargin == 5 && ! exist (outdir, "dir"))
     error ("__xzip__: OUTDIR output directory does not exist");
   endif