Get rid of CanonPath::fromCwd

As discussed in the last Nix team meeting (2024-02-95), this method
doesn't belong because `CanonPath` is a virtual/ideal absolute path
format, not used in file systems beyond the native OS format for which a
"current working directory" is defined.

Progress towards #9205

src/libutil/canon-path.cc

@@ -20,11 +20,6 @@ CanonPath::CanonPath(const std::vector<std::string> & elems)
         push(s);
 }
 
-CanonPath CanonPath::fromCwd(std::string_view path)
-{
-    return CanonPath(unchecked_t(), absPath(path));
-}
-
 std::optional<CanonPath> CanonPath::parent() const
 {
     if (isRoot()) return std::nullopt;