git download: Remove redundant argument in 'gexp->derivation' call.

* guix/git-download.scm (git-fetch): Call 'gexp->derivation' with only one
'#:local-build?' keyword argument.

guix/git-download.scm

@@ -109,8 +109,7 @@ HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
                       #:hash-algo hash-algo
                       #:hash hash
                       #:recursive? #t
-                      #:guile-for-build guile
-                      #:local-build? #t)))
+                      #:guile-for-build guile)))
 
 (define (git-version version revision commit)
   "Return the version string for packages using git-download."