doc: remove obsolete docs/images dir

[lukpueh]

Fixes #

Description of the changes being introduced by the pull request:
Remove obsolete docs/images directory which contains unused
variants of the logo. The canonical location of TUF logos is
theupdateframework/artwork, which has high-resolution formats (png
and svg) for all variants of the logo.

Also see theupdateframework/artwork#3.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[coveralls]

Pull Request Test Coverage Report for Build 1818311848

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 98.572%

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Totals
Change from base Build 1812636722:	0.2%
Covered Lines:	1094
Relevant Lines:	1106

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💛 - Coveralls

[jku]

Agreed, these do not seem to be used anywhere. There's plenty of TUF assets in https://github.com/cncf/artwork/tree/master/projects/tuf so no need to keep these here "just in case".