delete-active-region is a variable defined in `simple.el'.
Its value is

Whether single-char deletion commands delete an active region.
This has an effect only if Transient Mark mode is enabled, and
affects `delete-forward-char' and `delete-backward-char', though
not `delete-char'.

If the value is the symbol `kill', the active region is killed
instead of deleted.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.