GDScript: Remove unused values from `Token`

[HolonProduction]

• Token.source only usage was to initialize Token.identifier which we can do directly.
• We only use Token.cursor_place but not Token.cursor_position.

Size of Token on my PC:

• before: 72 bytes
• after removing the unused values: 64 bytes
• when placing the Variant at the end: 56 bytes (in my setup Variant has an alignment of 8 requiring additional padding when the values before it don't add to a multiple of 8)