build.py: avoid deleting taint files when writing stamps

The stamp cleaning process that occurs before writing out new stamps for
a task was deleting taint files as well. This resulted in tasks that
were forcibly re-executed using the -f or -C command line options to
have their previous output restored from shared state when called upon a
second time, because the taint value was no longer incorporated into the
task signature and thus it was reverting to its previous value. This
also affected the kernel menuconfig command in OE-Core.

Note that the taint file *is* still deleted when doing -c clean, which
is the desired behaviour.

Fixes [YOCTO #3919].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>