Always check -Wno-unused-but-set-variable gcc support

The previous commit 8a7e1ceefa wasn't
quite right.  This check applies to both the user and kernel space
build and as such we must make sure it runs regardless of what
the --with-config option is set too.

For example, if --with-config=kernel then the autoconf test does
not run and we generate build warnings when compiling the kernel
packages.

config/user.m4

@@ -9,6 +9,5 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_LIBBLKID
 	ZFS_AC_CONFIG_USER_LIBSELINUX
 	ZFS_AC_CONFIG_USER_FRAME_LARGER_THAN
-	ZFS_AC_CONFIG_USER_NO_UNUSED_BUT_SET_VARIABLE
 	ZFS_AC_CONFIG_USER_STACK_GUARD
 ])