Remove dead imports in figure.py left over from SubplotManager extraction #758

Trivial cleanup left over from #698. Three imports in ultraplot/figure.py are now referenced only by their own import statements — the code that used them moved to ultraplot/_subplots.py:

• from numbers import Integral (line 9)
• import matplotlib.gridspec as mgridspec (line 20)
• import matplotlib.projections as mproj (line 21)

$ grep -n '\bIntegral\b\|\bmgridspec\b\|\bmproj\b' ultraplot/figure.py
9:from numbers import Integral
20:import matplotlib.gridspec as mgridspec
21:import matplotlib.projections as mproj

Safe to delete.