Revert "Merge branch 'jk/use-wunreachable-code-for-devs' into next"

This reverts commit b5c54fe, reversing
changes made to 4cd3354.

Clang used on osx CI seems to be too clever and complains about a
defensive programming like

	if (sigfillset(&all))
		die_errno("sigfillset");

as it knows that the call would never fail on that platform.

Author: gitster

config.mak.dev

@@ -39,7 +39,6 @@ DEVELOPER_CFLAGS += -Wunused
 DEVELOPER_CFLAGS += -Wvla
 DEVELOPER_CFLAGS += -Wwrite-strings
 DEVELOPER_CFLAGS += -fno-common
-DEVELOPER_CFLAGS += -Wunreachable-code
 
 ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare