[llvm-jitlink] Don't add process symbols to every JITDylib.

The addProcessSymbols function added a generator for process symbols to every
JITDylib in the session, but this is unhelpful default behavior (e.g. it will
cause the ORC runtime's definition of __cxa_atexit to be shadowed by the
process's definition for all JITDylibs except main).

Since the loadProcessSymbols function already added a generator to main we only
need to drop this function. Other JITDylibs wishing to resolve process symbols
can link against the main JITDylib by passing `-lmain`.