Fix compilation with pycairo 1.17.x

Do not redefine a thing which was always defined in the included header,
since that breaks when upstream changes how they do things.

This is a holdover from the initial python2 version, since pycairo
dropped the use of downstream definition of Pycairo_CAPI during the
python3 port: https://github.com/pygobject/pycairo/commit/727470e8a170bf355c9689826bb1db95f7c1389e

See https://github.com/pygobject/pycairo/commit/921541295439f812ac51a6cc6d8143a8f10814b4
for the upstream change that broke this misuse.
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