Remove -webkit-mask-box-image aliases

[jamesnw]

Summary

This removes the 5 -webkit-mask-box-image aliases as standalone properties, as they already exist as alternative names in the properties they are aliases for. See #22727 and #22718 for more background.

Test results and supporting details

The support data for the alternative names in the keys they are aliases for was already more precise than in the files that are being deleted.

• -webkit-mask-box-image-outset
  • alias for mask-border-outset
  • alternative_name set for Chrome and Safari
• -webkit-mask-box-image-slice
  • alias for mask-border-slice
  • alternative_name set for Chrome and Safari
• -webkit-mask-box-image-repeat
  • alias for mask-border-repeat
  • alternative_name set for Chrome and Safari
• -webkit-mask-box-image-source
  • alias for mask-border-source
  • alternative_name set for Chrome and Safari
• -webkit-mask-box-image-width
  • alias for mask-border-width
  • alternative_name set for Chrome and Safari

Related issues

Fixes #22727

[jamesnw]

@ddbeck or @Elchi3 Can you confirm the alternative names is what was meant by folding in "files into their standard names, similar to most of these aliases"?

[caugner]

LGTM, verified for all five properties that the data is already contained or more specific in the unprefixed data.