Remove shader mask for web on shimmer loading cookbok notice

[daniel-naegele]

As the ShaderMask should work again (flutter/flutter#44152), the notice that it would not work from the shimmer loading cookbok was removed

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[daniel-naegele]

@googlebot I signed it!

[sfshaza2]

LGTM

[sfshaza2]

The build breakage is not related to this PR. Forcing a merge.