[PROTOCOL] don't emit Approval in transferFrom

[d10r]

What & Why

Due to the way it's implemented, transferFrom does emit an Approval event when subtracting the amount being transferred from the allowance. That seems wrong.

Next: check ERC20 spec and reference implementations if this is indeed unexpected behaviour.
If so, check where this is coming from (ERC777 implementation?). Remove.

[d10r]

Current OZ ERC20 does it this way: _approve takes a bool argument emitEvent which is set to false if invoked by transferFrom.

OZ v3.4 didn't have this flag. We inherited the behaviour from there.

In OZ this changed from 5.0 onwards.
OZ having changed it confirms the hypothesis that we should change its behaviour.