elf: remove redundant code from _dl_dst_substitute

There are just two users of _dl_dst_substitute: one is expand_dst that
sets is_path argument to 0, another one is expand_dynamic_string_token.
The latter function also has just two users: one is _dl_map_object that
sets is_path argument to 0, another one is fillin_rpath that sets
is_path argument to 1 and name argument contains no ':'.

In any case (is_path && name[i] == ':') is always false and all code
depending on it can be safely removed.

* elf/dl-load.c (_dl_dst_substitute): Remove checks that is_path
is set and name contains ':', and all code depending on these checks.