MAINT-5232: Make LLHTTPClientAdapter not be an LLSingleton.

llhttpclientadapter_test.cpp starts its every test by explicitly instantiating
a local LLHTTPClientAdapter object. This is an abuse of LLSingleton, and if it
had been properly defined (private constructor), it should never have compiled.
Looked at the other way, though, every known reference to LLHTTPClientAdapter
instantiates a local object. Why did someone think it should be an LLSingleton
in the first place? Remove LLSingleton<> as a base class; remove llsingleton.h.
This makes llhttpclientadapter_test.cpp work just fine.
One might also question what value this class adds. It seems to do very little
-- but more significantly, the ONLY references in the source tree are its
declaration, definition and test. Nobody actually uses it anywhere.