health: Update README to remove .override [DO NOT MERGE]

The .override module does not have correct rc and
sepolicy file_contexts configured, so let's allow
OEMs to keep healthd. If an OEM does want to remove
healthd they can provide a device-specific
health service impl.

Test: none
Bug: 110228707
Change-Id: I9ba32ae366df4ba58994ef0e3f9eb57cab5626f7