Sanitize buffer display handling in calendar.el, a first step.

Sanitize buffer display handling in calendar.el, a first step.

* window.el (window-safely-shrinkable-p): Remove.

* calendar/calendar.el (calendar-generate-window): Only fit a
vertically combined window to its buffer.
(calendar-basic-setup): Call pop-to-buffer, preferably making a
new window at the bottom of the frame.  Remove the
split-height-threshold binding around call of
calendar-list-holidays, this must be handled separately.