remove {.this.} pragma, deprecated since 0.19 (#20201)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/overload/tselfderef.nim

@@ -1,20 +0,0 @@
-discard """
-action: compile
-"""
-
-# bug #4671
-{.experimental.}
-{.this: self.}
-type
-  SomeObj = object
-    f: int
-
-proc f(num: int) =
-  discard
-
-var intptr: ptr int
-intptr.f() # compiles fine
-
-proc doSomething(self: var SomeObj) =
-  var pint: ptr int
-  pint.f() # Error: expression '.(pint, "f")' cannot be called