Fix Fwindow_text_pixel_size and fit-frame-to-buffer.

Fix Fwindow_text_pixel_size and fit-frame-to-buffer.

* xdisp.c (last_max_ascent): Re-remove after erroneously
reintroducing it on 2013-11-30 and abolishing Dmitry's removal
from 2013-03-29.
(move_it_to): Re-remove reference to last_max_ascent.
(Fwindow_text_pixel_size): Add iterator's max_ascent and
max_descent here instead of calling line_bottom_y.  Fix
doc-string.

* window.el (fit-frame-to-buffer): Fix calculations for margins and
height constraints.