gitmodules: Drop dejagnu tracking branch

We recently moved Dejagnu to the upstream master branch's top commit.
Therefore there is no need to set a tracking branch to 'dejagnu-1.6.3'.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

.gitmodules

@@ -12,7 +12,7 @@
 [submodule "dejagnu"]
 	path = dejagnu
 	url = https://git.savannah.gnu.org/git/dejagnu.git
-	branch = dejagnu-1.6.3
+	branch = master
 [submodule "newlib"]
 	path = newlib
 	url = https://sourceware.org/git/newlib-cygwin.git