By removing custom copy constructor.

Functionality kinda superset of std:: counterpart.

Unused.

Part of https://github.com/harfbuzz/harfbuzz/issues/1648

pollutes gcc bots logs https://circleci.com/gh/harfbuzz/harfbuzz/85395

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Priority should be given to specific over dispatch.  Broke sanitize before.
This fixes it, by moving prioritization to the context implementation, since
the correct priority cannot be done in the dispatch implementation.  Done
for subset and sanitize only, which need it.

To remove a couple of unwanted wrapper methods

This reverts commit c7f366fb.

Don't know how it got to master!