Revert "Always remove selection after cursor up/down"

Revert "Always remove selection after cursor up/down"

This reverts commit 008a0825e8d67b9c0a3d31b906f04cd7b8fd18a8.

This commit was part of bug #12310. The goal was to remove the
selection when using "up" at the start of selection (so that the
cursor cannot move). Unfortunately, it creates navigation issues for
insets inside tabulars.

Since the fix was minor, the safest decision is to revert.

The part of ticket #12310 that resets selection with
char-forwars/backward when moving is not possible still works.

Fixes bug #12593.