gpio: mockup: tweak the Kconfig help text

gpio-mockup doesn't require SYSFS to be selected so drop that bit from
the Kconfig text.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -1640,8 +1640,7 @@ config GPIO_MOCKUP
 	select IRQ_SIM
 	help
 	  This enables GPIO Testing driver, which provides a way to test GPIO
-	  subsystem through sysfs(or char device) and debugfs. GPIO_SYSFS
-	  must be selected for this test.
+	  subsystem through sysfs (or char device) and debugfs.
 	  User could use it through the script in
 	  tools/testing/selftests/gpio/gpio-mockup.sh. Reference the usage in
 	  it.