Remove [Install] section from `tracker-extract.service`

Having WantedBy=default.target was triggering the service before the session environment variable are set in gnome-session.

This service relies on gvfs-daemon.service and by starting it before the environment variables are set, some important variables like KRB5CCNAME are missing.

This means that the daemon won't have access to it when trying to mount shared folders through the network, creating problems like requesting credentials when trying to gio mount a samba share, despite having a valid kerberos ticket on the machine.

Since this is a helper service that is meant to be controlled by tracker-miner-fs-3.service, the install section shouldn't exist, as it allows the service to enabled, meaning that its execution would be controlled by systemd.

This is similar to the changes already made to the tracker-miner-fs service in !283 (merged) due to gvfs#481.