test: drop dead code from app-tap/msgpackffi test

It appears due to improper conflict resolution after pushing the
following commits in the reverse order:

* 2b9ef8d1 lua: don't modify pointer type in msgpack.decode*
* 84bcba52 lua: keeping the pointer type in msgpackffi.decode()

Originally 84bcba52 (which should land first) fixes the msgpackffi
module and introduces the test_decode_buffer() function locally for the
msgpackffi test. Then 2b9ef8d1 fixes the msgpack module in the same
way, expands and moves the test_decode_buffer() function to
serializer_test.lua (to use in msgpack and msgpackffi tests both).

After changes made to push the commits in the reverse order, those
commits doing something weird around tests. However the resulting state
is different from the right one just in the dead function in
msgpackffi.test.lua.

Follows up #3926.

(cherry picked from commit ec324247)