Remove warnings_as_errors from rebar.config

When new major versions of Erlang/OTP are released, they invariably
cause deprecation warnings, and despite keeping up with fixing these
against release candidates, something always breaks at the final
release. Remove warnings_as_errors to avoid problems for applications
using Yaws as a dependency.

Note that in the rebar3-support branch, warnings_as_errors is used
only in the dev profile.

rebar.config

@@ -4,8 +4,7 @@
 
 {sub_dirs, ["rel"]}.
 
-{erl_opts, [{src_dirs, ["src", "examples/src"]},
-            warnings_as_errors]}.
+{erl_opts, [{src_dirs, ["src", "examples/src"]}]}.
 
 {port_env, [{"CFLAGS", "$CFLAGS -g -O3 -Wall"},
             {"x86_64", "CFLAGS", "$CFLAGS -m$ERLANG_ARCH"},