sync: remove legacy sync info API

The legacy fence/pt info API has been deprecated for a while. This
change removes it from headers, so remaining users will have to switch
to the modern API when they're re-compiled. The functions are still
provided by libsync.so and tests remain, so existing binaries should
continue to work. Eventually these will be removed too, though, once
it's reasonable to expect those binaries to have been recompiled.

Bug: 35326015
Test: make checkbuild
Test: adb shell dumpsys SurfaceFlinger --latency
Change-Id: Id086fafe37c2bc1cfdca4a21107bc9208ed61f89