Remove module source code from test-api-stubs-docs

This makes it impossible to add new @TestApi in modules, of which
there are none right now and we don't want any going forwards, either.

Bug: 170395679
Test: m test-api-stubs-docs-check-current-api
Change-Id: I527ca7b580350372bee72b65f6596a46eab2ce04
Merged-In: I527ca7b580350372bee72b65f6596a46eab2ce04