[FRR]there is no need to checkout master branch after compiling frr. (#3941)

Signed-off-by: wangshengjun <wangshengjun@asterfusion.com>

src/sonic-frr/Makefile

@@ -21,7 +21,6 @@ $(addprefix $(DEST)/, $(MAIN_TARGET)): $(DEST)/% :
 	git checkout $(FRR_BRANCH)
 	stg branch --delete $(STG_BRANCH)
 	git rev-parse --short HEAD | xargs git checkout
-	git checkout master
 	git branch -D $(FRR_BRANCH)
 	popd
 	mv $(DERIVED_TARGET) $* $(DEST)/