Revert "libunwind: fix building with llvm"

This reverts commit 0fb2bc9895.

The patch is now applied in the libunwind sources.  This means that for
the last couple of months at least, the patch has been breaking the
LLVM build rather than fixing it — a good demonstration of why we
generally avoid conditional patch application.

pkgs/development/libraries/libunwind/default.nix

@@ -1,7 +1,6 @@
 {
   stdenv,
   lib,
-  fetchpatch,
   fetchFromGitHub,
   autoreconfHook,
   buildPackages,
@@ -20,11 +19,6 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-MsUReXFHlj15SgEZHOYhdSfAbSeVVl8LCi4NnUwvhpw=";
   };
 
-  patches = lib.optional (stdenv.targetPlatform.useLLVM or false) (fetchpatch {
-    url = "https://github.com/libunwind/libunwind/pull/770/commits/a69d0f14c9e6c46e82ba6e02fcdedb2eb63b7f7f.patch";
-    hash = "sha256-9oBZimCXonNN++jJs3emp9w+q1aj3eNzvSKPgh92itA=";
-  });
-
   postPatch =
     if (stdenv.cc.isClang || stdenv.hostPlatform.isStatic) then
       ''