emacs: Remove the notmuch-show-toggle-body command (with "b" binding)

In the recent switch to a JSON-based emacs interface, RET now toggles
message visibility anywhere in the message, (rather than only on the
summary line). So we no longer need this separate "b" binding for this.

Additionally, the body toggle was implemented independently from RET,
so after hiding a message with "b" one could not make it visible with
RET. This confusing state is now no longer possible, (since the
:body-visible property is removed entirely).