Help SIM5320 Breakout Board construction for Raspberry Pi model B
That component is a solder jumper to enable optional antenna power. Meaning, it is a special footprint on your PCB that can be easily bridged with solder in order to make a connection. Alternatively, you can remove the solder the break the connection. Solder jumpers are commonly used for board configuration when they are a connection that will be rarely changed.

I haven't used Eagle in a while, but IIRC Sparkfun distributes an Eagle library which includes the above symbol and a footprint or two for solder jumpers. If you don't want to bother, you could also use a resistor in the schematic (0Ω, DNP) and set it as an 0402 or 0603 footprint. Then a zero ohm resistor or small bit of wire can be used. Making your own footprint is also an option.

See page 48 of the datasheet. It shows an application circuit for both an active and passive antenna. If you know you are only going to be using a passive antenna, you can eliminate the circuit beyond the solder jumper (all you need is the antenna connector and a tuning network, which the Adafruit schematic doesn't seem to include).

Regarding your other circuitry: if you aren't using the LiPo battery stuff, and no microphone, then removing those blocks is fine. As long as all your logic I/O is 3V3, then removing the level shifting is also fine.