regulator: dbx500-prcmu: Remove unused function dbx500_regulator_testcase()

There isn't any code present within the current kernel to
override this 'weak' function.  Besides returning '0', which
is never checked anyway, the whole function appears to be
superfluous.

Consequently fixes W=1 warning:

 drivers/regulator/dbx500-prcmu.c:113:27: warning: no previous prototype for ‘dbx500_regulator_testcase’ [-Wmissing-prototypes]
 113 | int __attribute__((weak)) dbx500_regulator_testcase(
 | ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200625163614.4001403-5-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>