[web] Remove in-repo agent documentation

[harryterkelsen]

Removes some AI-generated design docs that I added to the repo. As we try to learn best practices for agentic engineering, I think we shouldn't pollute the repo with a bunch of docs (which are already out of date, by the way) until we have more standardized practices for agentic context management/knowledge management.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request removes three markdown design documents from the repository: the FallbackFontService Design Document, the Image Decoding in Flutter Web document, and the Image Decoding Throttling in Flutter Web document. There are no review comments, and I have no feedback to provide.

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autosubmit label was removed for flutter/flutter/190326, because - The status or check suite Dashboard Checks has failed. Please fix the issues identified (or deflake) before re-applying this label.