Remove comment citing fixed bug

One of the two issues preventing us from running test/api-digester/stability-stdlib-abi.swift in swift-evolve tests has been fixed; delete the comment referencing it. There’s another bug that still needs to be fixed, so we still aren’t running this yet.

Author: beccadax

test/api-digester/stability-stdlib-abi.swift

@@ -7,8 +7,6 @@
 // RUN: %clang -E -P -x c %t.tmp/changes.txt -o - | sed '/^\s*$/d' | sort > %t.tmp/changes.txt.tmp
 // RUN: diff -u %t.tmp/stability-stdlib-abi.swift.expected %t.tmp/changes.txt.tmp
 
-// The digester hasn't learned that we've stopped baking non-stored class member
-// order into the ABI. rdar://problem/46617463
 // The digester can incorrectly register a generic signature change when
 // declarations are shuffled. rdar://problem/46618883
 // UNSUPPORTED: swift_evolve