x/website: unify playground.js location

[bradfitz]

We currently have two copies of the playground's playground.js: one in x/tools (the original) and one in x/website (the new home, but currently unused?).

Unify.

/cc @ysmolsky @andybons @dmitshur @katiehockman @cnoellekb

[andybons]

The one in x/website is used by the playground widget on golang.org.

[dmitshur]

This task is a part of the larger "fully stripping down the godoc binary so that it only displays local code documentation" task in #29206 (comment).

Edit: Created issue #32011 for tracking that more specifically.

[dmitshur]

We should remove the one in x/website, as it's newer and unused. A better new home would be inside x/playground, but that can be a separate issue.

This caused confusion in CL 252377.

Edit: Not completely unused, I see #32007 (comment) above now.