selftests: mptcp: Remove the deprecated config NFT_COUNTER

NFT_COUNTER was removed since
390ad4295aa ("netfilter: nf_tables: make counter support built-in")
LKP/0Day will check if all configs listing under selftests are able to
be enabled properly.

For the missing configs, it will report something like:
LKP WARN miss config CONFIG_NFT_COUNTER= of net/mptcp/config

- it's not reasonable to keep the deprecated configs.
- configs under kselftests are recommended by corresponding tests.
So if some configs are missing, it will impact the testing results

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ma Xinjian <xinjianx.ma@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>