Clear weeds

[igrep]

There're many excessive exports and unnecessary dependencies, detected by weeder.
Let's delete really unnecessary ones.

[igrep]

Ref: The output of weeder at 35c9554:

> weeder .
= Package direct-hs-examples =

== Section exe:login exe:nippo exe:observe-pretty exe:ping ==
Module reused between components
* Common
Weeds exported
* Common
  - exitError

== Section exe:observe-pretty ==
Excessive other-modules entry
* Common

= Package network-messagepack-rpc =
No warnings

= Package network-messagepack-rpc-websocket =

== Section test:spec ==
Module not compiled
* Network.MessagePack.RPC.Client.WebSocketSpec
* Spec.hs
Redundant build-depends entry
* QuickCheck
* async
* bytestring
* data-msgpack
* envy
* hspec
* network-messagepack-rpc
* network-messagepack-rpc-websocket
* skews
* text
* wss-client

= Package direct-hs =

== Section library ==
Redundant build-depends entry
* unordered-containers
Weeds exported
* Web.Direct.Client
  - getCurrentDomain
  - setCurrentDomain
* Web.Direct.Client.Channel.Types
  - channelTalkId
* Web.Direct.Client.Status
  - Inactive
* Web.Direct.DirectRPC
  - apiVersion
* Web.Direct.DirectRPC.Map
  - decodeDomain
  - decodeUser
* Web.Direct.Message
  - encodeFile

= Package wss-client =

== Section exe:wss-client-sample ==
Module not compiled
* sample.hs
Redundant build-depends entry
* bytestring
* wss-client

== Section test:wss-client-test ==
Module not compiled
* Network.WebSockets.ClientSpec
* Spec.hs
Redundant build-depends entry
* bytestring
* envy
* hspec
* skews
* text
* wss-client

= Package skews =

== Section test:mock-ws-server-test ==
Module not compiled
* Network.WebSockets.SkewsSpec
* Spec.hs
Redundant build-depends entry
* async
* bytestring
* deque
* envy
* hspec
* network
* skews
* websockets