remove fedora 28 because it is eol

End of life as of 2019-05-28: https://fedoramagazine.org/fedora-28-end-of-life/

Signed-off-by: Andrew Hsu <andrewhsu@docker.com>
(cherry picked from commit 235feec)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 8069f72d763c6c3f38dbeae6ab3c72a8ff7e77d3
Component: packaging

Author: andrewhsu

components/packaging/rpm/Makefile

@@ -39,7 +39,7 @@ RUN?=$(RPMBUILD) rpmbuild-$@/$(ARCH) $(RPMBUILD_FLAGS)
 SOURCE_FILES=engine.tgz cli.tgz docker.service docker.socket plugin-installers.tgz
 SOURCES=$(addprefix rpmbuild/SOURCES/, $(SOURCE_FILES))
 
-FEDORA_RELEASES := fedora-31 fedora-30 fedora-29 fedora-28
+FEDORA_RELEASES := fedora-31 fedora-30 fedora-29
 CENTOS_RELEASES := centos-7
 DISTROS := $(FEDORA_RELEASES) $(CENTOS_RELEASES)