Yes, you can leave it out. But it certainly adds flavor to the mousseline cream. You can add other flavors such as coffee or chocolate to the mousseline cream. However, you can also easily substitute praline paste with nut butters too. It's not 100% identical because the sugar is not caramelized, and the nuts are not usually roasted, but it'll still work. For every 100 g of praline paste, substitute with 65 g 100% nut butter, such as peanut butter or almond butter or hazelnut butter, AND add 35 g of extra sugar to your pastry cream.