fix: remove plugin directory handle without fail handles greater than the removed one.

currently, remove a handle will fail all other handles which greater than the removed one, this will happen when more than one plugindirectory script running within a overlapped time.
it's a obvious bug since the handle supposed not always be 0, there is possibility following example happens:

ui make a plugin dir listing request, which assigned handle 0:
    plugin with handle 0 begin
    json-rpc make another plugin dir listing request, which assigned handle 1:
        plugin with handle 1 begin
    plugin with handle 0 end, in the current code, the handle will be erased
        plugin with handle 1 call function like setEndOfDirectory but handle 1 does not existed, the globalHandles vector size is 1 now the CPluginDirectory object with original handle 1 now on the handle 0 position, so the setEndOfDirectory call will failed, it's a mistake, this commit fix this problem. 

btw, I'm writing addons, but have no machine to compile xbmc, so please test it first before merge to the main branch.