* tramp-sh.el (tramp-remote-process-environment): Remove HISTFILE
and HISTSIZE; it's too late to set them here.  Add :version entry.
(tramp-open-shell): Add "HISTSIZE=/dev/null" to the shell's env
arguments.  Do not send extra "PSx=..." commands.
(tramp-maybe-open-connection): Setenv HISTFILE to /dev/null.