[HatoholArmPluginGate] Replace a wrong timeout value for SIGTERM

TIMEOUT_PLUGIN_TERM_SIGKILL_MS was wrongly used for it.

This commit fixes the following compile time warning:

HatoholArmPluginGate.cc:53:21: warning: unused variable 'TIMEOUT_PLUGIN_TERM_SIGTERM_MS' [-Wunused-const-variable]
static const size_t TIMEOUT_PLUGIN_TERM_SIGTERM_MS =  60 * 1000;