rebase, sequencer: remove the broken GIT_QUIET handling

rebase, sequencer: remove the broken GIT_QUIET handling

The GIT_QUIET environment variable was used to signal the non-am
backends that the rebase should perform quietly.  The preserve-merges
backend does not make use of the quiet flag anywhere (other than to
write out its state whenever it writes state), and this mechanism was
broken in the conversion from shell to C.  Since this environment
variable was specifically designed for scripts and the only backend that
would still use it is no longer a script, just gut this code.

A subsequent commit will fix --quiet for the interactive/merge backend
in a different way.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>