Revert "Makefile.extrawarn: turn on missing-prototypes globally"

This reverts commit 0fcb7085.

Too many warnings (actually errors because -Werror is enabled) are being
raised because of this.  After an effort to fix many of them, it became
apparent that there are also issues in some external modules used in
pre-submit which need repairing separately.  The aim is to reapply this
once all of the issues have been quashed.

Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I4400f42ffa9a176beef328f3a109f97a45643b15