Plotter: remove unnecessary type check
Plotter: remove unnecessary type check

The `$sectors` passed to `calculatePortToPortDistances` can only be an
array of SmrSector, so it cannot be null. We can safely remove this
check.

smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,hemberger/smr,smrealms/smr,hemberger/smr