mips: alchemy: remove historic comment on gpio build constraints

In ./arch/mips/alchemy/common/gpiolib.c, the comment points out certain
build constraints on CONFIG_GPIOLIB and CONFIG_ALCHEMY_GPIO_INDIRECT.

The commit 832f5dac ("MIPS: Remove all the uses of custom gpio.h")
makes all mips machines use the common gpio.h and removes the config
ALCHEMY_GPIO_INDIRECT. So, this makes the comment in alchemy's gpiolib.c
historic and obsolete, and can be removed after the commit above.

The issue on the reference to a non-existing Kconfig symbol was identified
with ./scripts/checkkconfigsymbols.py. This script has been quite useful
to identify a number of bugs with Kconfig symbols and deserves to be
executed and checked regularly.

So, remove the historic comment to reduce the reports made the script and
simplify to use this script, as new issues are easier to spot when the
list of reports is shorter.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>