Remove warnings_as_errors from rebar.config

rebar's beam files are actually compiled by bootstrap's call to
make:files/2, so warnings_as_errors in rebar.config isn't really used.

rebar.config

@@ -11,8 +11,7 @@
 %% type name depending of the OTP version used.
 {erl_opts,
  [
-  {platform_define, "^[0-9]+", namespaced_types},
-  warnings_as_errors
+  {platform_define, "^[0-9]+", namespaced_types}
  ]}.
 
 {xref_checks, []}.