[gdb/tui] Remove default entries in TUI translation tables

[gdb/tui] Remove default entries in TUI translation tables

The TUI translation tables contain default entries at the end:
...
static struct tui_translate tui_border_kind_translate_hline[] = {
  { "space",    ' ' },
  { "ascii",    '-' },
  { "acs",      -1 },
  { 0, 0 },
  { "ascii",    '-' }
};
...

A simpler way of implementing this would be to to declare the first (or last)
entry the default, but in fact these default entries are not used.

Make this explicit by removing the default entries, and asserting in translate
that an entry will always be found.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>