Revert "Windows extract: always chmod files (JuliaIO#85)"

This reverts commit 3ff3375.

src/extract.jl

@@ -74,17 +74,15 @@ function extract_tarball(
         elseif hdr.type == :file
             read_data(tar, sys_path, size=hdr.size, buf=buf)
             # change executable bit if necessary
-            tar_exec = !iszero(0o100 & hdr.mode)
-            sys_exec = Sys.isexecutable(sys_path)
-            if tar_exec != sys_exec
+            if Sys.iswindows() ⊻ !iszero(0o100 & hdr.mode)
                 mode = filemode(sys_path)
-                if tar_exec
+                if Sys.iswindows()
+                    # turn off all execute bits
+                    mode &= 0o666
+                else
                     # copy read bits to execute bits with
                     # at least the user execute bit on
                     mode |= 0o100 | (mode & 0o444) >> 2
-                else
-                    # turn off all execute bits
-                    mode &= 0o666
                 end
                 chmod(sys_path, mode)
             end