tink-prf: drop unused test file
tink-prf: drop unused test file

For integration tests, Rust runs any .rs file found as a test
crate of its own, so there's no need for a mod.rs

Change-Id: I9ffa5bac6b948a403cfb9bf3664749e40339bdcf

project-oak/tink-rust,project-oak/tink-rust,project-oak/tink-rust