examples: Remove deprecated rmtchar example application

The upper-half RMT driver is no longer available on NuttX. Instead,
Espressif's RMT peripheral was bound directly to the lirc driver.
For testing purposes, use the `irtest` application. NuttX OS PR
apache/nuttx#18654 removed the upper-half
driver interface used by this application, so removing it does not
break any existing feature on NuttX.

Signed-off-by: Tiago Medicci <tiago.medicci@espressif.com>

Author: tmedicci