fix: drop the self-assignment at the top of ContextInstanceMixin.get_current

[darkdi]

get_current opens with cls.__context_instance = cls.__context_instance. __init_subclass__ already sets that attribute on every subclass, so the read and the write hit the same entry in the class dict and the line is a no-op. It reads as if it were priming something, which is what sent me looking.

[github-actions]

❌ Changelog is required!

You need to add a brief description of the changes to the CHANGES directory.

Changes file should be named like <issue or PR number>.<category>.rst,
example 1234.bugfix.rst where 1234 is the PR or issue number and bugfix is the category.

The content of the file should be a brief description of the changes in
the PR in the format of a description of what has been done.

Possible categories are: feature, bugfix, doc, removal and misc.

[darkdi]

Fragment is in as CHANGES/1895.misc.rst, but changes-required looks like it cannot go green on any fork PR at the moment: actions/checkout refuses to check out fork code under pull_request_target unless the step sets allow-unsafe-pr-checkout: true, so the job dies before towncrier runs