Revert "Workaround for missing updated libgcc.i686"

This is no longer needed since we're relaxed dependency in gcc
(fffee0c0eb5c749a8b687d13fad12944b480b7be in qubes-gcc repository).
This reverts commit b79f692.

Fixes QubesOS/qubes-issues#3650

Makefile.builder

@@ -11,9 +11,6 @@ else ifeq ($(PACKAGE_SET),vm)
   endif
 endif
 
-ifeq ($(DIST),fc25)
-    SOURCE_PREP := workaround-gcc-upgrade-fc25
-endif
 
 source-debian-xen-copy-in: VERSION = $(shell cat $(ORIG_SRC)/version)
 source-debian-xen-copy-in: ORIG_FILE = "$(CHROOT_DIR)/$(DIST_SRC)/xen_$(VERSION).orig.tar.gz"
@@ -22,6 +19,3 @@ source-debian-xen-copy-in:
 	-$(ORIG_SRC)/debian-quilt $(ORIG_SRC)/series-debian-vm.conf $(CHROOT_DIR)/$(DIST_SRC)/debian/patches
 	tar xfz $(SRC_FILE) -C $(CHROOT_DIR)/$(DIST_SRC)/debian-vm --strip-components=1 
 	tar cfz $(ORIG_FILE) --exclude-vcs --exclude=debian -C $(CHROOT_DIR)/$(DIST_SRC)/debian-vm .
-
-workaround-gcc-upgrade-fc25:
-	sudo chroot $(CHROOT_DIR) dnf install -y gcc-6.4.1-1.qubes1.fc25.x86_64 libgcc.x86_64