Remove shade50 getter from MaterialAccentColor

[Reekoooo]

This PR removes shade50 getter from MaterialAccentColor class. It is not part of Material design 2014 specs and calling it causes null pointer exception at runtime.

List which issues are fixed by this PR. You must list at least one issue.
Fix #93270

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• 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.
• All existing and new tests are passing.

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[HansMuller]

LGTM