[css-overflow-3] -webkit-line-clamp need not support a <'block-ellipsis'> value

[heycam]

https://drafts.csswg.org/css-overflow-3/#webkit-line-clamp

The new definition of -webkit-line-clamp looks good, but now that we are not trying to make it a pure alias for line-clamp, we should not support parsing the <'block-ellipsis'> value in it. This isn't supported by implementations.

[frivoal]

Closed by merging: fd2db4a