set null prompt function in dumb term mode

The erl prompt is displayed when I start iex with dumb terminal mode.
I evaded it by changing a default prompt function of erlang.

before:
bash-3.2$ TERM=DUMB bin/iex
Warning: could not run smart terminal, falling back to dumb one
Eshell V6.2  (abort with ^G)
1> 1> 1> 1> Interactive Elixir (1.1.0-dev) - press Ctrl+C to exit (type h() ENT\
ER for help)
1> iex(1)> 1> 
after:
bash-3.2$ env TERM=DUMB bin/iex
Warning: could not run smart terminal, falling back to dumb one
Eshell V6.2  (abort with ^G)
Interactive Elixir (1.1.0-dev) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)>