[gdb/tui] Remove local CTRL_CHAR definition

In gdb/tui/tui-io.c we have a local definition of CTRL_CHAR:
...
 /* Use definition from readline 4.3.  */
 #undef CTRL_CHAR
 #define CTRL_CHAR(c) \
      ((c) < control_character_threshold && (((c) & 0x80) == 0))
...

We require readline >= 7.0, so we can expect the definition to be available.

Remove the superfluous local definition.

Tested on x86_64-linux.

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