textview: Match entry behavior for backspace
As pointed out in #3417 (closed), there is a long-standing difference in how GtkEntry and GtkTextView treat Ctrl-Shift-Backspace (and other variations): GtkEntry always operates on the selection first, if it exists. GtkTextView only handled plain Backspace that way, and ignores the selection for other variations.
There is no good reason for this difference, so just remove it and make GtkTextView behave the same as GtkEntry.
Fixes: #3417 (closed)