Remove explain-phase assertion which is in fact reachable

[Zac-HD]

Fixes #3755, by deleting some defensive code which I wrote to defend against implementation bugs and thought was unreachable in normal use. Obviously that turned out to be wrong! So, how can you reach that?

The relevant loop is replacing an infix of our test buffer, for example taking AA XX BB and trying AA YY BB. However, the length can vary by contents, so the test might try to read off the end of the buffer: AA YYB B-. In this case, we'll try fixing it up as AA YYB BB - and my incorrect assert was that we'd have exactly this buffer from the rerun. However, it's possible for the infix to first overflow, and then have part of the infix dropped by the canonicalisation logic (e.g. because of a filter) - so the final buffer is actually (e.g.) AA YB BB. This is in fact working as intended, so deleting the oversensitive check is a complete solution 😅