Revert "Show U+FFFD REPLACEMENT CHARACTER for invalid Unicode codepoints"

We now handle U+FFFD replacement in hb_buffer_add_utf*().  Any other
manipulation can happen in user callbacks.  No need for this.

https://github.com/behdad/harfbuzz/commit/efe74214bbb68eaa3d7621e73869b5d58210107e#commitcomment-7039404

This reverts commit efe74214.

Conflicts:
	src/hb-ot-shape-normalize.cc

src/hb-ot-shape-normalize.cc

@@ -209,14 +209,7 @@ decompose_current_character (const hb_ot_shape_normalize_context_t *c, bool shor
   else if (decompose_compatibility (c, buffer->cur().codepoint))
     skip_char (buffer);
   else
-  {
-    /* Not found, not decomposible;  If codepoint is invalid Unicode and
-     * font supports U+FFFD REPLACEMENT CHARACTER, use that instead. */
-    hb_codepoint_t FFFD_glyph;
-    if (buffer->cur().codepoint > 0x10FFFFu && c->font->get_glyph (0xFFFDu, 0, &FFFD_glyph))
-      glyph = FFFD_glyph;
     next_char (buffer, glyph); /* glyph is initialized in earlier branches. */
-  }
 }
 
 static inline void