Remove unused code in Dataset#hash_key_symbol
Remove unused code in Dataset#hash_key_symbol

Originally, I planned to use this to implement the auto-aliasing
for single arguments in get/select_map/select_order_map.  I ended
up using a different for that, and nothing uses this, so it should
be safe to remove.

offshore-safety/sequel,reubano/sequel,bigfix/sequel,chef/sequel,pfarrell/sequel,beni55/sequel,freshpay/sequel,mpalmer/sequel,offshore-safety/sequel,reubano/sequel,davidbegin/sequel,t-morgan/sequel,pfarrell/sequel,nicohvi/sequel,uhoh-itsmaciek/sequel,davidbegin/sequel,mpalmer/sequel,seeARMS/sequel,uhoh-itsmaciek/sequel,seeARMS/sequel,knut2/sequel,gferguson-gd/sequel,yazshel/sequel,nicohvi/sequel,gferguson-gd/sequel,freshpay/sequel,nju520/sequel,dennis-d/custom_sequel,yazshel/sequel,t-morgan/sequel,offshore-safety/sequel,Marketcircle/sequel,knut2/sequel,dennis-d/custom_sequel,beni55/sequel,nju520/sequel