[SDP-872] Removes required attribute from Shipping Categories dropdown
[SDP-872] Removes required attribute from Shipping Categories dropdown

There was a problem with select2 dropdown and HTML validation on iOS
devices. Validation messages were not styled and sometimes didn't show
up at all. Validation is still performed by backend and proper error
messages are being shown.

imella/spree,imella/spree,imella/spree