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 efe74214bbb68eaa3d7621e73869b5d58210107e.

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