elf: remove redundant code from is_dst

is_dst is called either by _dl_dst_count or by _dl_dst_substitute.
_dl_dst_count is called by DL_DST_COUNT only.
DL_DST_COUNT is called either by expand_dst with is_path == 0
or by expand_dynamic_string_token.
_dl_dst_substitute is called either from expand_dst with is_path == 0
or from expand_dynamic_string_token.
The latter function is called either from _dl_map_object with is_path == 0
or from fillin_rpath with is_path == 1 and name containing 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 (is_dst): Remove checks that is_path is set and name
contains ':', and all code depending on these checks.