common.sh: drop unused DEFAULT_FAST

The last user was removed in CL:261260, so clean it up.

BUG=None
TEST=CQ passes

Change-Id: Id42954526dc305107af538b0de7aca49f5f818c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1769240
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/common.sh b/common.sh
index cb1f2d1..2064c93 100644
--- a/common.sh
+++ b/common.sh
@@ -372,9 +372,6 @@
   fi
 fi
 
-# Enable --fast by default.
-DEFAULT_FAST=${FLAGS_TRUE}
-
 # Directory to store built images.  Should be set by sourcing script when used.
 BUILD_DIR=