devextreme-angular: clean:src-metadata on regenerate

[mpreyskurantov]

Patch for:

• chore(devextreme-angular): migrate from Gulp to NX (part 2) #32006

Fixes a subj for a new (after NX) folders / files structure, where:

\devextreme-angular\
  - \src\
    -- \metadata\ (<<-- src sub)

Before NX it was:

\devextreme-angular\
  - \metadata\ (<<-- src sibling)
  - \src\

https://github.com/DevExpress/DevExtreme/blob/25_1/packages/devextreme-angular/gulpfile.js#L30-L39
https://github.com/DevExpress/DevExtreme/blob/25_1/packages/devextreme-angular/build.config.js#L3-L12

[Copilot]

Pull request overview

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