Revert "Handle bot_message on on_message method"
Revert "Handle bot_message on on_message method"

This reverts commit 8a7212d3b44319b55c0f459d6ba510dbfd5b91c5, It was
unnecessary.

Fixup #18

YasuhiroKinoshita/ruboty-slack_rtm,rosylilly/ruboty-slack_rtm,tsuwatch/ruboty-slack_rtm,kinoppyd/ruboty-slack_rtm