Hi Sunil,
The fields like BP & SECCO cannot be made mandatory by Field status group. You have to use either Validations(OB28) or you can use SHD0(screen transaction variant).
Please check in your system how the fields are made mandatory.
Secondly, you don't need to remove this Validation as you would still be needing this for other BP & SECCO.
You can simply write a Validation to issue error message when the discontinued BP & SECCO are used by USers. This will stop the entry in future with this BP & SECCO and others can continue as usual.
Regards,
Kavita