selftests: Remove KSFT_TAP_LEVEL

Since sub-testing can now be detected by indentation level, this removes
KSFT_TAP_LEVEL so that subtests report their TAP header for later parsing.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>