Do not set input focus in i3-input. (#2598)

Do not set input focus in i3-input. (#2598)

This commit removes all traces of setting and reverting the input focus
in i3-input. We don't need to do this because grabbing the keyboard is
sufficient to have the attention we need.

Changing the input focus and reverting it can cause situations where i3
executes the IPC command before processing the FocusIn events. This leads
to i3's input focus change to be rejected due to the timing, leading to
an inconsistent focus state.

fixes #2597