Remove schema facility and most of formats

[lukpueh]

• addresses part of #731
• blocks on #773, #774, #775 (commits are included in this PR)

close #183

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The usage of the securesystemslib schema/formats facility has been little by little phased out and is now nowhere used anymore. Several reasons to not use it are listed in #183.

This patch removes schema.py along with all securesystemslib schema definitions in formats.py.

Additionally, it removes unrelated and unused helper functions in formats.py.

[lukpueh]

Some windows runners seem to calculate test coverage differently than the rest and thus drop below the already lowered constraint. I'll fix this, when this PR is no longer blocked by other PRs.

[lukpueh]

Some windows runners seem to calculate test coverage differently than the rest and thus drop below the already lowered constraint.

9be1638 further lowers the coverage requirement. As before, this does not mean that there are fewer tests (see commit message).

[jku]

this does not mean that there are fewer tests (see commit message).

Some of the bad coverage is quite expected (aws, azure, gcp, sigstore). I was surprised that HSMSigner is the worst though -- I wonder what that's about.

[lukpueh]

this does not mean that there are fewer tests (see commit message).

Some of the bad coverage is quite expected (aws, azure, gcp, sigstore). I was surprised that HSMSigner is the worst though -- I wonder what that's about.

securesystemslib/.github/workflows/_test.yml

Lines 62 to 64 in 37089c5

	elif [ "$RUNNER_OS" == "Windows" ]; then
	echo "Skipping HSM tests on Windows"
	# see https://github.com/secure-systems-lab/securesystemslib/issues/520