.inputrc: Disable the system bell (\a) for the Readline library

I enable the system bell (xset b) to get IRC notifications from irssi,
but I don't want Readline beeping at me if I try and move outside the
line ;).

For docs on other available settings, see [1].

[1]: https://www.gnu.org/software/bash/manual/html_node/Readline-Init-File-Syntax.html