Remove Function::is_call

The documentation says that this is only used for heuristics, but it
is never actually called. This should be removed for now and perhaps
added back later if we find an actual use for it.

src/lib.rs

@@ -847,10 +847,6 @@ pub trait Function {
     /// Get the block parameters for a given block.
     fn block_params(&self, block: Block) -> &[VReg];
 
-    /// Determine whether an instruction is a call instruction. This is used
-    /// only for splitting heuristics.
-    fn is_call(&self, insn: Inst) -> bool;
-
     /// Determine whether an instruction is a return instruction.
     fn is_ret(&self, insn: Inst) -> bool;