remove inserted derefs for ref object fields when transforming to dot call (#24498)

fixes #24492

Kind of a goofy way of doing this, but we count how many derefs were
used for the first parameter before calling `builtinFieldAccess`, then
count after, and if there are more now than before, we remove the added
derefs. I thought maybe getting rid of #18298 would simplify it but
maybe this would still be the best way.

For better encapsulation we could make `dotTransformation` take an
`nOrig` param instead but this would be less efficient since it would
need a copy, though `semAsgn` already makes one.

tests/objects/trefobjfieldoverload.nim

@@ -0,0 +1,6 @@
+block: # issue #24492
+  type MyType = ref object
+    a: int
+
+  proc a(val: MyType, i: int) = discard
+  MyType().a(100)