Drop builtin-features from eval.scm

[sjamaan]

This seems like it's a hack from ye olden days when we didn't have a module system yet, and for example (require srfi-30) would otherwise try to attempt loading a unit library named srfi-30.

This is confusing and a bit weird, and also there's ##sys#features which might run out of sync with this features list.

To make this change, the srfi module registrations should be changed to be as part of the library unit. For example by changing register-primitive-module to use #f or 'library as the module library name.

NOTE: Check what the situation with chicken.csi and chicken.foreign is - can they also use #f or 'library?

[sjamaan]

I checked, and actually (require srfi-30) did give an error in 4.13. Also, ​SRFI-55 doesn't expand to require but to import in CHICKEN 5, so I think it won't get broken by this change, either.

[sjamaan]

Was fixed as part of 137ae4adb