tree-wide: Convert to using autoptr(GString) vs g_string_free(...,TRUE)

If we're freeing the segment, it's basically always better to use
`autoptr()`.  Fewer lines, more reliable, etc.

Noticed an instance of this in the pull code while reviewing a different PR,
decided to do a grep for it and fix it tree wide.

Closes: #836
Approved by: pwithnall

src/libostree/ostree-bootloader-grub2.c

@@ -148,7 +148,7 @@ _ostree_bootloader_grub2_generate_config (OstreeSysroot                 *sysroot
                                           GError                       **error)
 {
   gboolean ret = FALSE;
-  GString *output = g_string_new ("");
+  g_autoptr(GString) output = g_string_new ("");
   g_autoptr(GOutputStream) out_stream = NULL;
   g_autoptr(GPtrArray) loader_configs = NULL;
   guint i;
@@ -260,8 +260,6 @@ _ostree_bootloader_grub2_generate_config (OstreeSysroot                 *sysroot
 
   ret = TRUE;
  out:
-  if (output)
-    g_string_free (output, TRUE);
   return ret;
 }