Remove erroneous ELF note.

On ELF platforms we currently output an ELF note but with totally invalid
contents.  I tried experimenting to see whether it was possible to do it
properly and tie the result into ReflectionContext, but it turns out to be a
hard problem because of issues to do with cross-segment symbol references and
relocations.

The upshot is that I think for now it's best just to remove it.

We might revisit this in future at some point; it would be good to be able to
reliably find the Swift metadata just from the ELF Phdrs.

rdar://101749382

Author: al45tair

stdlib/public/runtime/SwiftRT-ELF.cpp

@@ -83,14 +83,3 @@ static void swift_image_constructor() {
 
   swift_addNewDSOImage(&sections);
 }
-
-__asm__(".section \".note.swift_reflection_metadata\", \"aw\"");
-
-static __attribute__((__used__))
-__attribute__((__section__(".note.swift_reflection_metadata")))
-__attribute__((__aligned__(1)))
-struct {
-  const char MagicString[sizeof(SWIFT_REFLECTION_METADATA_ELF_NOTE_MAGIC_STRING)];
-  const swift::MetadataSections *Sections;
-} __attribute__((__packed__))
-Note = {SWIFT_REFLECTION_METADATA_ELF_NOTE_MAGIC_STRING, &sections};