fix(build): remove the extra cowlib deps from emqx_auth_http

apps/emqx_auth_http/rebar.config

@@ -1,6 +1,5 @@
 {deps,
- [{cowlib, {git, "https://github.com/ninenines/cowlib", {tag, "2.7.0"}}},
-  {gun, {git, "https://github.com/emqx/gun", {tag, "1.3.4"}}},
+ [{gun, {git, "https://github.com/emqx/gun", {tag, "1.3.4"}}},
   {gproc, {git, "https://github.com/uwiger/gproc", {tag, "0.8.0"}}}
  ]}.