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 <[e-mail]>
Signed-off-by: Ma Xinjian <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/mptcp/config

···
 CONFIG_NETFILTER_ADVANCED=y
 CONFIG_NETFILTER_NETLINK=m
 CONFIG_NF_TABLES=m
-CONFIG_NFT_COUNTER=m
 CONFIG_NFT_COMPAT=m
 CONFIG_NETFILTER_XTABLES=m
 CONFIG_NETFILTER_XT_MATCH_BPF=m