samples: led_strip: fix bbc_microbit.conf location

Commit 55d9d1c6
("samples: led: consolidate LED samples under same directory")
moved the led_strip sample from samples/drivers to samples/drivers/led.

But commit f54a53b4
("drivers: ws2812_gpio: Make timing configurable and less hardware
dependend") was merged at the same time and added the bbc_microbit.conf
file to the old location.

This patch moves bbc_microbit.conf to its correct location.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>