Remove check for address zero in ERC1155.balanceOf

[frangio]

We have a require statement in this function that doesn't seem to be mandated ERC-1155, or at least I can't find it in the spec. It looks like it should be removed.

openzeppelin-contracts/contracts/token/ERC1155/ERC1155.sol

Lines 70 to 71 in 1642b66

	function balanceOf(address account, uint256 id) public view virtual override returns (uint256) {
	require(account != address(0), "ERC1155: address zero is not a valid owner");

[Amxx]

This might have been imported from the ERC-721 standard into the ERC-1155 implementation (for consistency)

/// @dev NFTs assigned to the zero address are considered invalid, and this
/// function throws for queries about the zero address.

(IMO, the part of ERC-721 is really bad)

[balajipachai]

In that case it's better we remove it, and update the tests wherever required.