malloc: Remove unused tcache code from unsorted bin scan

malloc: Remove unused tcache code from unsorted bin scan

Now that fastbins have been removed, there is no need to add chunks
to tcache during an unsorted scan.  Small blocks can only be added
to unsorted as a result of a remainder chunk split off a larger block,
so there is no point in checking for additional chunks to place in
tcache.  The last remainder is checked first, and will be used if it
is large enough or an exact fit.  The unsorted bin scan becomes simpler
as a result.  Remove the tcache_unsorted_limit tunable and manual entries.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>