Remove redundant SetProcessingMode(InitProcessing) calls

Remove redundant SetProcessingMode(InitProcessing) calls

After several refactoring iterations, auxiliary processes are no
longer initialized from the bootstrapper. Using the InitProcessing
mode for initializing auxiliary processes is more appropriate. Since
the global variable Mode is initialized to InitProcessing, we can just
remove the redundant calls of SetProcessingMode(InitProcessing).

Author: Xing Guo <higuoxing@gmail.com>
Discussion: https://www.postgresql.org/message-id/CACpMh%2BDBHVT4xPGimzvex%3DwMdMLQEu9PYhT%2BkwwD2x2nu9dU_Q%40mail.gmail.com