fix: force-delete-pool event is not empty

[zhongwencool]

(emqx@x)77> gproc_pool:delete(ehttpc:name(emqx_modules_acl_http)).
** exception error: not_empty
     in function  gproc_pool:call/1 (gproc_pool.erl, line 636)
(emqx@x)78> gproc_pool:delete(ehttpc:name(emqx_modules_supper_http)).
** exception error: not_empty
     in function  gproc_pool:call/1 (gproc_pool.erl, line 636)
(emqx@x)79> gproc_pool:force_delete(ehttpc:name(emqx_modules_supper_http)).
true
(emqx@x)82> gproc_pool:force_delete(ehttpc:name(emqx_modules_acl_http)).
true
(emqx@x)83> gproc_pool:force_delete(ehttpc:name(emqx_modules_auth_http)).
true

Sometimes the pool can't be deleted when is not empty. we must force_delete it.

[zmstone]

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