ls-tree: remove path filtering logic in show_tree

ls-tree: remove path filtering logic in show_tree

ls-tree uses read_tree_recursive() which already does path filtering
using pathspec. No need to filter one more time based on prefix
only. "ls-tree ../somewhere" does not work because of
this. write_name_quotedpfx() can now be retired because nobody else
uses it.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>