Aztec.nr: Remove assert in `is_valid_impl`

[rahul-kothari]

Allows devs to assert based on the return value rather than being forced to fail.

Consider removing this line:

aztec-packages/noir-projects/noir-contracts/contracts/schnorr_hardcoded_account_contract/src/main.nr

Line 46 in 5ac5788

assert(verification == true);