plug-ins: fix #8508 Print exporting to PSD seems to export as cm ...

and import as inches

When GIMP's display unit was mm, our PSD export converted the X and Y
resolution as if a conversion from mm to inch was needed.

However, PSD's resolution is always measured in pixels per inch, so a
conversion is never necessary.
Removing the conversion, resolves the issue.

plug-ins/file-psd/psd-save.c

@@ -834,18 +834,15 @@ save_resources (GOutputStream  *output,
 
     if (g_unit == GIMP_UNIT_MM)
       {
-        gdouble factor = gimp_unit_get_factor (g_unit) / 10.0;
-
-        xres /= factor;
-        yres /= factor;
-
         psd_unit = PSD_UNIT_CM;
       }
     else
       {
         psd_unit = PSD_UNIT_INCH;
       }
 
+    /* Don't convert resolution based on g_unit which is a display unit.
+     * PSD resolution is always in pixels/inch. */
     xres_fix = xres * 65536.0 + .5; /* Convert to 16.16 fixed point */
     yres_fix = yres * 65536.0 + .5; /* Convert to 16.16 fixed point */