Remove hiddenapi-mappings and hiddenapi-list-docs.

We still need to keep access to removed APIs to keep them on greylist. Instead of having a separate run to generate those, we would instead depend on the actual droidstubs.

Kudos to Makoto for producing build traces:

"m droid"
Before: https://screenshot.googleplex.com/oEcd4x76YAz
After: https://screenshot.googleplex.com/CxZCNJMDyK5

"m sync"
Before: https://screenshot.googleplex.com/7cjE96qcNAL
After: https://screenshot.googleplex.com/po6BhCoP9L3

Bug: 143864733
Test: diff out/soong/hiddenapi/hiddenapi-flags.csv
Change-Id: If9dcac94eb077c6265c5acf5b3ce6d262db64f90