julia: remove redundant Nix-specific mtime patch

As far as I can tell this patch is redundant as all pre-compiled code
generated at build time is baked into the Julia system image and will
thus never get invalidated: Note that for both julia_10 and julia_15
there are no `.ji` files produced in the derivations.

pkgs/development/compilers/julia/1.5.nix

@@ -34,12 +34,6 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./use-system-utf8proc-julia-1.3.patch
-
-    # Julia recompiles a precompiled file if the mtime stored *in* the
-    # .ji file differs from the mtime of the .ji file.  This
-    # doesn't work in Nix because Nix changes the mtime of files in
-    # the Nix store to 1. So patch Julia to accept mtimes of 1.
-    ./allow_nix_mtime.patch
   ];
 
   postPatch = ''