[khmer] Remove more unused code

Prodded by https://github.com/harfbuzz/harfbuzz/pull/2583

These are leftovers from when we forked Khmer shaper from the Indic shaper.
diff --git a/src/hb-ot-shape-complex-khmer.cc b/src/hb-ot-shape-complex-khmer.cc
index 11c79b2..dddba14 100644
--- a/src/hb-ot-shape-complex-khmer.cc
+++ b/src/hb-ot-shape-complex-khmer.cc
@@ -141,27 +141,6 @@
 
 struct khmer_shape_plan_t
 {
-  bool get_virama_glyph (hb_font_t *font, hb_codepoint_t *pglyph) const
-  {
-    hb_codepoint_t glyph = virama_glyph;
-    if (unlikely (virama_glyph == (hb_codepoint_t) -1))
-    {
-      if (!font->get_nominal_glyph (0x17D2u, &glyph))
-	glyph = 0;
-      /* Technically speaking, the spec says we should apply 'locl' to virama too.
-       * Maybe one day... */
-
-      /* Our get_nominal_glyph() function needs a font, so we can't get the virama glyph
-       * during shape planning...  Instead, overwrite it here.  It's safe.  Don't worry! */
-      virama_glyph = glyph;
-    }
-
-    *pglyph = glyph;
-    return glyph != 0;
-  }
-
-  mutable hb_codepoint_t virama_glyph;
-
   hb_mask_t mask_array[KHMER_NUM_FEATURES];
 };
 
@@ -172,8 +151,6 @@
   if (unlikely (!khmer_plan))
     return nullptr;
 
-  khmer_plan->virama_glyph = (hb_codepoint_t) -1;
-
   for (unsigned int i = 0; i < ARRAY_LENGTH (khmer_plan->mask_array); i++)
     khmer_plan->mask_array[i] = (khmer_features[i].flags & F_GLOBAL) ?
 				 0 : plan->map.get_1_mask (khmer_features[i].tag);