chore: Disable tsc sitemap generation

[mattmspie]

The sitemaps included with npm packages are broken because they point to .ts source files, which are not distributed. Until a final decision is made as to whether .ts source should be distributed, disable tsc's sitemap generation.

[mattmspie]

Google CLA signed.

This is meant to be a stop-gap for #5638 until the team decides whether sourcemaps + .ts source should be included in npm packages. Hence, not adding Fixes #... line to end of commit message.

[anlexN]

can we solve PR?

[stefan-jonasson]

@mattmspie @asyncLiz Any updates on this? It would be really great if this could be merged. Right now the broken source-maps really complicate things downstream and because they are broken they do not add any value for anyone.