Move polar panel-border blanking into themeable.py

[iangow]

Upstream PR review item from has2k1#1059.

Source review thread:

• Added two new coordinate systems: coord_polar and coord_radial. has2k1/plotnine#1059 (comment)

Requested change:

• Move the polar spine blanking logic out of ggplot.py and into plotnine/themes/themeable.py.
• Implement the behavior in the panel_border themeable, for example by checking whether "polar" in ax.spines and hiding ax.spines["polar"] when blanking the panel border.

Review goal: keep theme behavior in themeables instead of adding coordinate-specific theme handling in ggplot.py.

[iangow]

Addressed by commit b11b891: Move polar border blanking to themeable