//visibility:override is no longer needed for impl_library_visibility
to override visibility.

Removing this allows the defaults module to specify better defaults.

- Stub libraries are made publicly visible, via `visibility`
- Impl libraries are private by default, but visibility is extended
  by the modules

Bug: 165017290
Test: m
Exempt-From-Owner-Approval: build refactor
Change-Id: Ibf35bfac5c99a21125f89ba10945f3364217b90f