lncli: remove confusing default 0 for time lock delta

[C-Otto]

Note: depends on #6777

Change Description

This changes the output of lncli updatechanpolicy --help from

--time_lock_delta value  the CLTV delta that will be applied to all forwarded HTLCs (default: 0)

to

--time_lock_delta value  the CLTV delta that will be applied to all forwarded HTLCs (default: no default)

This fixes #1523

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[Roasbeef]

Overlap with #6762