Remove the uses of {.procvar.} pragma (#14359)

This pragma did nothing.

Ref:
- https://github.com/nim-lang/Nim/issues/2172#issuecomment-383276469
- https://github.com/nim-lang/Nim/issues/12975

lib/pure/rationals.nim

@@ -225,7 +225,7 @@ proc `/=`*[T](x: var Rational[T], y: T) =
   x.den *= y
   reduce(x)
 
-proc cmp*(x, y: Rational): int {.procvar.} =
+proc cmp*(x, y: Rational): int =
   ## Compares two rationals.
   (x - y).num