Remove extra over-mixing checks

[abbrazi]

When processing DEP1 instructions in the ErrorAnalyzer, p > 0.75 is checked in:

a) undo_DEPOLARIZE1 and
b) depolarize1_probability_to_independent_per_channel_probability

Likewise, p > 0.9375 is checked in:

c) undo_DEPOLARIZE2 and
d) depolarize2_probability_to_independent_per_channel_probability

I removed checks (a) and (c). I also added tests for checks (b) and (d).

Let me know if this was intentional.

Signed-off-by: Abbas Bracken Ziad abbas.ziad25@imperial.ac.uk

[Strilanc]

Good eye and the change looks correct but unfortunately the redundant check was intentional, in order to give a better error message.

Python users will more easily understand what to do in response to Can't analyze over-mixing DEPOLARIZE1 errors (probability > 3/4). than when told depolarize1_probability_to_independent_per_channel_probability with p>0.75.