ras-page-isolation: drop an unused variable

ras-page-isolation: drop an unused variable

There's no need to store the value of strtoul() during the
overflow check. Remove it, as this is causing a warning:

ras-page-isolation.c: In function ‘parse_isolation_env’:
ras-page-isolation.c:166:47: warning: unused variable ‘converted_value’ [-Wunused-variable]
  166 |                                 unsigned long converted_value = strtoul(config->env, &endptr, 10);
      |                                               ^~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>