index: remove error message in non-error remove

If `git_index_remove_bypath` does no work, and returns an OK error
code, it should not set an error message.

src/index.c

@@ -1173,6 +1173,9 @@ int git_index_remove_bypath(git_index *index, const char *path)
 		ret != GIT_ENOTFOUND))
 		return ret;
 
+	if (ret == GIT_ENOTFOUND)
+		giterr_clear();
+
 	return 0;
 }