tests: remove an XFAIL: linux for a test which does not crash on linux anymore

Author: eeckstein

test/SILGen/collection_cast_crash.swift

@@ -1,7 +1,5 @@
 // RUN: %target-swift-frontend -O -primary-file %s -emit-sil -o - | FileCheck %s
 
-// XFAIL: linux
-
 // check if the compiler does not crash if a function is specialized
 // which contains a collection cast