selftests: user: remove user suite

The user test suite has only one test, test_user_copy which loads
test_user_copy module for testing. But test_user_copy module has already
been converted to kunit (see fixes). Hence remove the entire suite.

Fixes: cf6219ee ("usercopy: Convert test_user_copy to KUnit test")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>