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

tests/generics/t6137.nim

@@ -16,7 +16,7 @@ let
   # define a vector of length 3
   x: vector[3] = [1.0, 3.0, 5.0]
 
-proc vectFunc[T](x: vector[T]): vector[T] {.procvar.} =
+proc vectFunc[T](x: vector[T]): vector[T] =
   # Define a vector function
   result = 2.0*x