Remove excess generality from the bitmap sweep walking routine.

The bitmaps that are passed to the sweep walking routine always have
the same base, bitsLen, and max values.  As such, there is no need to
account for situations where these values may be different.  This is
guarnateed because bitmaps being swept are actually private copies of
the global bitmaps which are initialized with common parameters.

Furthermore, the sweep callback does not change the max values, so we
do not need a clean-up loop to accomodate changes to max.  The sweep
routine operates on garbage objects and therefore has no need to set
bits in a bitmap and, by extension, update the max value.

Also eliminated was an obsolete TODO regarding the unrolling of a
loop.  We will leave that class of optimization to the compiler.

Change-Id: Iff6ede3d0ac68f88d381a06911fb6bd9cada926d