don't open source svg for writing

while working on a setup where fontcustom was used in a context where it doesn't have write-permission on the source images, I noticed that it's still opening the source files for writing.

In-fact, I believe that `removeSwitchFromSvg` was at some point working on the original file, but later was switched to act on a temporary file.

As such, both the `r+` read mode and the `seek(0)` on the source file were not needed at all.

By changing to just `r` mode (and removing the unneeded `seek`), fontcustom now doesn't need write-access to the source images any more.