Remove ACDS 13.1 req from i2c component

This should permit builds on older Quartuses again.

This may be the least-distasteful of three possibilities:

1) Generate the IP cores with the oldest version of Quartus we wish to support (bleh)
2) Find some vaguely-documented feature that allows us to turn off this version signature (bleh)
3) Hand-edit a "DO NOT MODIFY" file (bleh)