Remove reloc_block

It isn't called and all reloc sinks either ignore it or panic when it is
called.

cranelift/codegen/src/binemit/mod.rs

@@ -140,9 +140,6 @@ pub trait CodeSink {
     /// Add 8 bytes to the code section.
     fn put8(&mut self, _: u64);
 
-    /// Add a relocation referencing a block at the current offset.
-    fn reloc_block(&mut self, _: Reloc, _: CodeOffset);
-
     /// Add a relocation referencing an external symbol plus the addend at the current offset.
     fn reloc_external(&mut self, _: SourceLoc, _: Reloc, _: &ExternalName, _: Addend);