Revert part of 503c7c16: InsetMathNest:edit resets anchor.

Revert part of 503c7c16: InsetMathNest:edit resets anchor.

Fixes bug reported here:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg217265.html

1. Start a new document.
2. Start a math inset and inside put "1+2".
3. Put the cursor to the right of "+".
4. Press <Shift> + L three times. This will select the entire math inset
   (as expected).
5. Press <Shift> + R.

Before 503c7c16, the selection inside the math inset was preserved.
After 503c7c16, it is lost.

The problem is that InsetMathNest does too much for our needs and
resets the anchor (why, I do not know). Therefore, it is better to
duplicate the code.