remove conflicting default call in tables.getOrDefault (#24265)

fixes #23587

As explained in the issue, `getOrDefault` has a parameter named
`default` that can be a proc after generic instantiation. But the
parameter having a proc type [overrides all other
overloads](f73e03b132/compiler/semexprs.nim (L1203))
including the magic `system.default` overload and causes a compile error
if the proc doesn't match the normal use of `default`. To fix this, the
`result = default(B)` initializer call is removed because it's not
needed, `result` is always set in `getOrDefaultImpl` when a default
value is provided.

This is still a suspicious behavior of the compiler but `tables` working
has a higher priority.

(cherry picked from commit 67ea754b7f)

tests/stdlib/ttables.nim

@@ -314,3 +314,18 @@ block countTableWithoutInit:
   d.inc("f")
   merge(d, e)
   doAssert d["f"] == 7
+
+block: # issue #23587
+  type
+    A = proc ()
+
+  proc main =
+    let repo = initTable[uint32, A]()
+
+    let c1 = repo.getOrDefault(uint32(1), nil)
+    doAssert c1.isNil
+
+    let c2 = repo.getOrDefault(uint32(1), A(nil))
+    doAssert c2.isNil
+
+  main()