Remove LLVMRustCoverageHashCString

Coverage has two FFI functions for computing the hash of a byte string. One
takes a ptr/len pair, and the other takes a NUL-terminated C string.

But on closer inspection, the C string version is unnecessary. The calling-side
code converts a Rust `&str` into a C string, and the C++ code then immediately
turns it back into a ptr/len string before actually hashing it.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -373,11 +373,6 @@ pub(crate) fn write_mapping_to_buffer(
     }
 }
 
-pub(crate) fn hash_str(strval: &str) -> u64 {
-    let strval = CString::new(strval).expect("null error converting hashable str to C string");
-    unsafe { llvm::LLVMRustCoverageHashCString(strval.as_ptr()) }
-}
-
 pub(crate) fn hash_bytes(bytes: &[u8]) -> u64 {
     unsafe { llvm::LLVMRustCoverageHashByteArray(bytes.as_ptr().cast(), bytes.len()) }
 }