INSTALL: remove duplicate sentence

The same sentence is repeated in the next paragraph.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>

INSTALL

@@ -225,7 +225,6 @@ XEN_BUILD_TIME=hh:mm:ss
 SMBIOS_REL_DATE=mm/dd/yyyy
 VGABIOS_REL_DATE="dd Mon yyyy"
 
-During tools build external repos will be cloned into the source tree.
 This variable can be used to point to a different git binary to be used.
 GIT=