Revert "kill_buffer: go to about:blank instead of erroring when killing last buffer"

This reverts commit db2a0551ab556535177bc2be670be919b280b44e.

Patch is not quite right, as kill_buffer_hook will not get run, so, for
example, sessions won't be updated.  Also, we can't call 'load' if we
don't know what type of buffer this is, because not all buffer types have
that method.  This will need to be reworked.