Remove extra difficulty check for proofs

[tjsturos]

There was a redundant check for if the increment was over 800k.

This would be redundant due to the first check of "if the difficulty was under 25k" would be met at anything over 700k.

Moved to a function, due to it being used twice (in the Calculating and Verifying of the Proof) and to be able to add tests.

[CassOnMars]

looks good minus the underflow check