Delete check for program licenses directory

[d-torrance]

This is another short followup to #3240. I forgot to delete a line that was left over from when copyright was a string, which is no longer the case.

Also, the next thing after the then at the end of the line was the definition of the path variable. So if the "program licenses" directory doesn't exist (e.g., in the Debian/Ubuntu package where we don't vendor any 3rd party programs or libraries), then we immediately got the following error:

/usr/share/Macaulay2/Core/startup.m2:583:16:(0): error: expected argument 1 to be a list or sequence

This was from a call to join, which expects lists and sequences, but argument 1 was path, which was still just a symbol.