[Pinning] Finalize the configuration about pinning

[giomfo]

The pinning dimension has been introduced here (#24)

After thinking the fingerprints of the certificates won't be whitelisted in the withPinning variant, only the root certificat will be imposed thanks to the network_security_config.xml

This network_security_config.xml is already defined and correct for the withPinning variant here

We request here:

• apply the network_security_config of Element-Android to the withoutPinning variant. We should "git move" this xml file there
• remove the PinningConfiguration.kt files from all variants, because the defined items are not used anymore