Clean src before building to remove abandoned files

[desrosj]

This addresses an issue where files persist in the src directory of the build server checkout despite having been removed. Because the files are not under version control and there's no explicit delete command, they are not removed from src as expected.

Specific details about this issue can be found in this comment.

Now that grunt clean has been updated to account for the new files and locations, it can be run before building to ensure a clean state (see dc20352).

This is a short-term solution that's meant to run once and be removed. The long-term solution is the work being done in #11064, which will re-add these files to version control, which solves this problem naturally through changes to the files themselves.

Trac ticket: Core-64393.

Use of AI Tools

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Props desrosj.

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[aaronjorbin]

Looks and sounds good to me

[desrosj]

Merged in r62069.