rebar.config: delete unused and unimplemented 'app_bin' option

app_bin option was used by the install command which has been
removed in 51ed787.

rebar.config

@@ -1,7 +1,6 @@
 %% -*- mode: erlang;erlang-indent-level: 4;indent-tabs-mode: nil -*-
 %% ex: ts=4 sw=4 ft=erlang et
 
-{app_bin, ["priv/rebar"]}.
 %% escript_incl_extra is for internal rebar-private use only.
 %% Do not use outside rebar. Config interface is not stable.
 {escript_incl_extra, [{"priv/templates/*", "."}]}.

rebar.config.sample

@@ -138,9 +138,6 @@
 
 %% == OTP Applications ==
 
-%% Binaries to link into the erlang path?
-{app_bin, []}.
-
 %% Enable validation of the OTP app module list. Default is 'true'
 {validate_app_modules, true}.