tools: frr-reload fix list value not present

tools: frr-reload fix list value not present

Check for value present in list before removing
as in certain python3 ValueError traceback is observed.

Traceback (most recent call last):
  File "/usr/lib/frr/frr-reload.py",
		line 2278, in <module>
    (lines_to_add, lines_to_del, restart_frr)
	= compare_context_objects(newconf, running)
  File "/usr/lib/frr/frr-reload.py",
		line 1933, in compare_context_objects
    lines_to_add, lines_to_del
  File "/usr/lib/frr/frr-reload.py",
		line 1549, in ignore_delete_re_add_lines
    lines_to_del.remove((ctx_keys, line))
ValueError: list.remove(x): x not in list

Ticket:#3389979
Issue:3389979

Testing Done:
With fix perform frr-relaod on frr.conf config where earlier
traceback was seen.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Chirag Shah <chirag@nvidia.com>