selftests: remove duplicate include

'assert.h' included in 'sparsebit.c' is duplicated.
It is also included in the 161th line.
'string.h' included in 'mincore_selftest.c' is duplicated.
It is also included in the 15th line.
'sched.h' included in 'tlbie_test.c' is duplicated.
It is also included in the 33th line.

Link: https://lkml.kernel.org/r/20210316073336.426255-1-zhang.yunkai@zte.com.cn
Signed-off-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/kvm/lib/sparsebit.c

@@ -1890,7 +1890,6 @@ void sparsebit_validate_internal(struct sparsebit *s)
  */
 
 #include <stdlib.h>
-#include <assert.h>
 
 struct range {
 	sparsebit_idx_t first, last;