audio: Remove redundant PrintTo definitions

hidl-gen now generates them for use by all Google Tests, calling
toString().

The local audio PrintTo definitions in PrettyPrintAudioTypes.h are now
redundant with the generated versions, and the entire header can be
removed.

Test: Verified the generated PrintTo functions work in a different GTest
Change-Id: I80d100907e3c6c638608b0b8b34232b02decac19