misc/cgo/fortran: clean up after successful helloworld.90 build

Change-Id: If03185a21e088c9b383a82c14d649805f00c8042
Reviewed-on: https://go-review.googlesource.com/20000Reviewed-by: Minux Ma <minux@golang.org>

misc/cgo/fortran/test.bash

@@ -13,6 +13,7 @@ if ! $FC helloworld/helloworld.f90 -o main.exe >& /dev/null; then
   echo "skipping Fortran test: could not build helloworld.f90 with $FC"
   exit 0
 fi
+rm -f main.exe
 
 if ! go test; then
   echo "FAIL: go test"