selftests/bpf: Clean up dynptr prog_tests

Clean up prog_tests/dynptr.c by removing the unneeded "expected_err_msg"
in the dynptr_tests struct, which is a remnant from converting the fail
tests cases to use the generic verification tester.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230214051332.4007131-2-joannelkoong@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>