Fix Stormshield

[kphoenix137]

This one may be a bit controversial. Based on: diasurgical/devilution#64 (comment)

By removing INVCURS from unique items, Stormshield now has a slot open that it didn't before.

In the PR demo, Stormshield had 50% lightning resistance with no set graphic. I was looking around other item data from the PR demo comparing it to retail to see what the "correct" lightning resist amount should be.

Here's what I found
In the PR demo all the unique item resistance values were identical to retail resistance values, with some notable exceptions:

• Aguinara's Hatchet: 100% resist magic -> 80% resist magic
• Schaefer's Hammer: 100% resist lightning -> 80% resist lightning
• Inferno: 75% resist fire -> 80% resist fire
• Constricting Ring: 100% resist all -> 75% resist all

It appears that previous resistances were capped at 100% in the PR demo, and the cap was likely revised to 80% and then 75%. All resistance items that were not at 75% or higher went untouched, leading me to believe that 50% lightning resist is the correct amount for Stormshield in retail.