build_image: don't force tpm use flag for factory images

Don't add tpm to USE flags when building factory images

BUG=chrome-os-partner:55836
TEST=build_image --board=kevin-tpm2 factory_install

Change-Id: I5d41e3b3e0e66d5d4e8ba0cb76302630dfed1686
Reviewed-on: https://chromium-review.googlesource.com/365510
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
diff --git a/build_image b/build_image
index ddd6cbf..0feeae1 100755
--- a/build_image
+++ b/build_image
@@ -136,7 +136,7 @@
 
   BASE_PACKAGE="virtual/target-os-factory-shim"
 
-  export USE="${USE} fbconsole vtconsole factory_shim_ramfs tpm i2cdev vfat"
+  export USE="${USE} fbconsole vtconsole factory_shim_ramfs i2cdev vfat"
 fi
 
 # TODO: </prebuild hook>