Refactoring #25302; don't store procedure's parameter types to PType.sonsImpl

[demotomohiro]

No description provided.

[demotomohiro]

Current code generates compile error from following code:

proc map[T](op: proc (): T) =
  discard op()

proc f(): string = "foo"
map(f)

mytests/testgene.nim(5, 4) template/generic instantiation of `map` from here
mytests/testgene.nim(1, 12) Error: invalid type: 'T' in this context: 'proc (op: proc (): T{.closure.})' for proc

Edit: Fixed the bug and it compiles without error.