rm_work.bbclass: use HOSTTOOLS 'rm' binary exclusively

The do_rm_work() task is using the first available 'rm' binary
available in PATH to remove files and folders.
However, depending on the PATH setup and RECIPE_SYSROOT_NATIVE
contents, the function can be using the 'rm' binary available
in RECIPE_SYSROOT_NATIVE, a folder that will get removed.
This causes a sporadic race-condition when trying to access the
'rm' binary of a folder already deleted.
Solve this by exclusively using the HOSTTOOLS 'rm' binary, as
this folder will not get removed.

Signed-off-by: Luis Martins <luis.pinto.martins@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit edcd9ad3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>