Remove SPL_LINUX_CONFIG autoconf macro

Since removing the check for CONFIG_PREEMPT, there are no consumers of
the SPL_LINUX_CONFIG macro. As such, there is no reason to keep it
around.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #164