elf: remove redundant is_path argument

is_path argument is no longer used and could be safely removed.

* elf/dl-dst.h (DL_DST_COUNT): Remove is_path argument, all callers
updated.
* elf/dl-load.c (is_dst, _dl_dst_count, _dl_dst_substitute,
expand_dynamic_string_token): Likewise.
* sysdeps/generic/ldsodefs.h (_dl_dst_count, _dl_dst_substitute): Remove
is_path argument.