pimd: in 'no ip pim hello' add hold time as optional when hello interval given

pimd: in 'no ip pim hello' add hold time as optional when hello interval given

Issue:
  User is allowed to configure only hello without hold timer but when undo
  config, the hold timer is mandatory as shown below:
  FRR-4(config-if)# ip pim hello 10
    <cr>
    (1-180)  Time in seconds for Hold Interval
  FRR-4(config-if)# ip pim hello 10
  FRR-4(config-if)# no ip pim hello 10
    (1-180)  Time in seconds for Hold Interval
  FRR-4(config-if)# no ip pim hello 10
  % Command incomplete: no ip pim hello 20

Fix:
  Making the hold timer as optional when undo config.

Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>