Remove internal srfi's

[Geert Janssens]

Coming back to an old thread... see
https://lists.gnucash.org/pipermail/gnucash-devel/2010-March/027641.html

The thread indicates that if our own sfri's are not pulled in when building 
against guile 1.6, they can be safely removed from our source.

The only platform that is still using guile 1.6 is Windows. (I tried moving 
the windows build to guile 1.8.x but failed. I have given up on that attempt 
for now).

I verified that none of our own srfi's are pulled in, so this means we can 
eliminate the local srfi's.

I'll probably do this in one of the following days.

Geert