Update product.js

The previous calcule was wrong : 
ex :
* TaxRate : 20
* Price : 120  tax inc.
old : 120 * ( 1/(1+20) / 100 ) = 0.05714
new : 120 / ( 1+(20/100) ) = 100

For the if statement : when shop is B2B, the priceWithDiscountsWithoutTax is already in tax exclude.

thanks to @Jitrixis
120 [tax incl] * ( 1/(1+20) / 100 ) = 0.05714 [tax excl]