selftests: drop KSFT_KHDR_INSTALL make target

Drop the KSFT_KHDR_INSTALL make target now that all use-cases have
been removed from the other kselftest Makefiles.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>