Remove useless variable spa_active_count

This isn't required for the Linux port because the kernel tracks
if a module is busy. The prototype for spa_busy() is also removed
since its definition was already removed.

Signed-off-by: Isaac Huang <he.huang@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3262