Remove `try_as_void_ptr_capsule` feature from smart_holder branch.

This manual removal was informed by reviews of these commits (newest to oldest):

* d930de0bca046774acf2cd0c09b1e4ef84d8c0bb
* 114c0f2a3e94b759e32dad897f25fc979ef79a43
* 1c10b097a79008c8fb0107fb61831126784f8b88
* 9d4b4dffce6677a7c9a9af37a45f4436d353f5e1
* da058a2904bb5eb1a82417bf36ab2357a1af4d3f
4 files changed