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/llvm/ffi.rs

@@ -1916,7 +1916,6 @@ extern "C" {
     );
 
     pub fn LLVMRustCoverageCreatePGOFuncNameVar(F: &Value, FuncName: *const c_char) -> &Value;
-    pub fn LLVMRustCoverageHashCString(StrVal: *const c_char) -> u64;
     pub fn LLVMRustCoverageHashByteArray(Bytes: *const c_char, NumBytes: size_t) -> u64;
 
     #[allow(improper_ctypes)]