Remove deprecated class `iris.fileformats.rules.FunctionRule`

[lbdreyer]

The class iris.fileformats.rules.FunctionRule was deprecated in Iris 1.10

https://github.com/lbdreyer/iris/blob/015cee9877df87d64e84146975feb9eefea84591/lib/iris/fileformats/rules.py#L482

[DPeterK]

Closed by #2782.