Clear context column for all assumed bits

[ekzyis]

@t-bast mentioned that #1242 should have cleared the context column of payment_secret in #1307 (comment):

But is this also why var_onion_optin does not have 9 in its context column, even though payment_secret does have 9? Both are assumed bits.

It's simply that when making payment_secret assumed in #1242, the context column wasn't cleared (while it was cleared for other features when they were marked assumed). We can clear the context column for payment_secret to make it more consistent with other assumed features, that would avoid confusion.

This PR clears the context column for ALL assumed bits to avoid confusion.