Remove getServiceProxy() on BufferHubBinderService

We didn't notice that binder has already defined a getService(name,
outref*) function to get a binder service when originnally writing
getServiceProxy(). Therefore, keep the function there is unnecessary.

Test: "atest buffer_hub_binder_service-test" passed.
Bug: 112338294
Change-Id: I8ec8e452f577867e0f345d61f096456fea1456ac