common/json_parse_simple: drop redundant and wrong json_str_to_u64()

[whitslack]

The json_str_to_u64() function contains incorrect logic. It chops one character off of the beginning and end of the JSMN token and then parses the remainder as a u64, but JSMN_STRING tokens already do not include the enclosing quotation marks, so json_str_to_u64() would actually parse the JSON string "1234" into the integer 23. Oops! Also note that it would simply fail on all input strings shorter than two characters since tok->end would wind up before tok->start.

Just drop the function entirely. It was only used in one place, and that place explicitly doesn't care whether its input is a JSON number or a numeric string, and it was already calling json_to_u64() as an alternative, and that function already accepts both JSON strings and JSON numbers as input, so the call to json_str_to_u64() would have been entirely redundant if it had been correct.

Changelog-Fixed: The keysend command no longer corrupts the type numbers of extra TLVs when they are specified as numeric strings longer than 2 digits.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes. I'm not confident that I know how to do this well, but I could take a crack at it if it's really necessary.
• Documentation has been reviewed and updated as needed. The logic was previously wrong, so presumably this fix brings the behavior in line with the existing documentation.
• Related issues have been listed and linked, including any that this PR closes. I didn't find any.

[vincenzopalazzo]

ACk 0681609