staging.bbclass: do not add extend_recipe_sysroot to prefuncs of prepare_recipe_sysroot

staging.bbclass: do not add extend_recipe_sysroot to prefuncs of prepare_recipe_sysroot

When running prepare_recipe_sysroot task, the extend_recipe_sysroot
is run twice.

What prepare_recipe_sysroot does is executing extend_recipe_sysroot,
there's no need to add extend_recipe_sysroot to its prefuncs.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>