AES-GCM cipher tag and IV size, while attempting to render encrypted ciphertext useless by removing them
But, for the purpose of optimization, I was thinking to delete only its IV and Tag, thus hoping to make decryption of that segment's data computationally infeasible.


Neither work.  The IV might seem to work (as doing a brute force search over a space of size $2^{96}$ might appear to be daunting), however the attacker has another potential approach.

If an attacker has the ciphertext, has a correct guess to 16 bytes of the plaintext, and the key, he can recover the IV; from there, he can recover the rest of the plaintext.

Because of the likelihood of the attacker guessing 16 bytes of plaintext, this wouldn't appear to be feasible.

And, the tag is even worse (and so is the AAD, in case that occurs to you); an attacker with the key, iv and ciphertext can trivially decrypt the ciphertext without the tag.  He might not be certain that the ciphertext hasn't been modified; however he might be willing to assume that it wasn't.

Since I gave you two negative answers, lets give you a positive idea; why don't you make the key generation procedure:

key :=SHA2( master key || extra data )


Where the extra data is stored on in the segment header (and nowhere else); it is generated randomly when you first encrypt; when you decide to do a quick erasure, you overwrite it. The master key is the one derived from the password with PBKDF2.

If this extra data is at least 256 bits long, recovering the key is no easier than attacking AES-256 directly.