Remove the hack for iframes using srcdoc + sandbox

[fungairino]

Currently we use a hack to remove the sandbox attribute from iframes that use the srcdoc and sandbox attribute in order for our content script to be properly injected into the frame. For more context on the hack, see: #8927 and https://pixiebrix.slack.com/archives/C07DQ2J7C78

Ideally we could just enable "match_origin_as_fallback": true for our content script configuration in the manifest.json, but we run into a chromium bug regarding dynamic imports within a sandbox frame.

This issue tracks removing the above-mentioned hack once the related chromium issues have been fixed:
https://issues.chromium.org/issues/355256366
https://issues.chromium.org/issues/40564567

The related end-to-end test should still pass once this hack is removed.

pixiebrix-extension/end-to-end-tests/tests/runtime/srcdocFrames.spec.ts

Line 24 in 1b2eb68

test("8143: mods can run in srcdoc iframes", async ({