gas/Dwarf5: drop dead code

Commit 3417bfca676f ("GAS: DWARF-5: Ensure that the 0'th entry in the
directory table contains the current... ") added a "dwarf_level < 5"
check to out_dir_and_file_list(). This rendered dead that branch of the
construct, due to the enclosing if()'s "DWARF2_LINE_VERSION >= 5".
Delete that code as well as the corresponding part of the comment.

While there also drop a redundant "dirs != NULL": "dirs" will always be
non-NULL when dirs_in_use is not zero.