Remove GCM mode from CI_Encryption (OpenSSL)

While openssl_get_cipher_methods() lists 'aes-<keysize>-gcm' as supported,
it appears that this is only half of the story. To be more specific, only
the encryption operation of GCM is performed, and the authentication message
is completely missing, rendering the whole thing useles.
Missing looping index added. Without index empty rows returned for csv column headers.
Merge pull request #3130 from dionysiosarvanitis/fix/pdo_result__list_fields

Missing looping index added.