[flutter_markdown] remove backgroundColor from code text theme to enable code selection highlight

[HosamHasanRamadan]

fixes: flutter/flutter#150053
related to : flutter/flutter#96112

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• 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.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[domesticmouse]

I'll defer to @stuartmorgan if this is the best approach.

[stuartmorgan]

I'll defer to @stuartmorgan if this is the best approach.

I'm not familiar with this portion of the framework, so I'm not a good authority for this. @HosamHasanRamadan I would suggest asking for feedback on this in the #hackers-framework Discord channel.