Remove obsolete DEBUG only instance variable BasicBlock::bbLoopNum

[briansull]

Contributes to #13582

Old comment at declaration in block.h

        // This is the label a loop gets as part of the second, reachability-based
        // loop discovery mechanism.  This is apparently only used for debugging.
        // We hope we'll eventually just have one loop-discovery mechanism, and this will go away.
        INDEBUG(loopNumber bbLoopNum;) // set to 'n' for a loop #n header

This was only used by some debug Dumping and some asserts that aren't necessary.
It was very confusing as we have another loop index that is important to the loop optimizer.
As it is used to index the natural loop table: optLoopTable[index]

    loopNumber bbNatLoopNum; // Index, in optLoopTable, of most-nested loop that contains this block,
                             // or else NOT_IN_LOOP if this block is not in a loop.

[briansull]

@dotnet/jit-contrib PTAL

[AndyAyersMS]

LGTM