Remove saved file's mtime check comparing it with the current time

Remove saved file's mtime check comparing it with the current time

As the edited file can be a remote file on a server with a different time
zone, the mtime can actually be in the future. In this case the check not
only shows the misleading warning but more importantly the

doc->priv->mtime < st.st_mtime

check never happens and the user doesn't get the modified file prompt.

Setting

doc->priv->mtime = time(NULL);

to the current time on file creation isn't harmful in any way because the
saved file's mtime is taken but it's a bit misleading so better to set it
to 0.