Although I wouldn't recommend editing that line in the core, here is how to remove the login message:
Find this section in CharacterHandlers.cpp and delete the Powered by....message, save the file a recompile the core.
Code:
#ifdef WIN32
_player->BroadcastMessage("delete this line", MSG_COLOR_WHITE, BUILD_TAG,
BUILD_REVISION, CONFIG, ARCH, MSG_COLOR_LIGHTBLUE);
#else
_player->BroadcastMessage("delete this line", MSG_COLOR_WHITE, BUILD_TAG,
BUILD_REVISION, PLATFORM_TEXT, ARCH, MSG_COLOR_LIGHTBLUE);
As for the Welcome to my server message you can make a more fancy and colourful message-on-join with a simple Lua Script and it will display across the screen not only in the chatbox.