feedv2: investigate du feed sizes fail to parse, data for none sizes, drop funky gender logic

[zhehaowang]

• gender can be supplied from stockx response.
• du feed with static size eu <-> us mapping seems not to work out for many models. Investigate why.
• du writes None.json for size None, this definitely seems funky.

[zhehaowang]

Sizing issue: it appears the only issues now are
du:

• child sizes (like 11C)
• Nike 49.0
• There seems two women size charts for Nike? (The Y and non-Y variants and the latter is not handled). And some Du listing shows women shoes with men's sizing chart (153265-018).
  stockx:
• W sizes, example: CD9705-406, https://stockx.com/air-jordan-10-retro-maya-moore-w