[gdb/tui] Delete line buffer when switching to singlekey

[gdb/tui] Delete line buffer when switching to singlekey

Say we're in TUI mode, and type "sun":
...
(gdb) sun
...

After switching to SingleKey mode using C-x s, we have just:
...
sun
...

After typing "d", we get:
...
sun
Undefined command: "sundown".  Try "help".
...

The SingleKey "d" is supposed run the "down" command.

Fix this by clearing the readline line buffer when switching to SingleKey
mode.

Tested on x86_64-linux.

PR tui/30522
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30522

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