[Flang][Tests] Remove GPL notice to GFortran test suite documentation.

Originally #161912, we've now decided that an explicit GPL notification is
redundant with the LICENSE file, which is a common convention for relaying
this information.

Author: Cameron McInally

flang/docs/FortranLLVMTestSuite.md

@@ -73,5 +73,3 @@ instructions described [above](#running-the-llvm-test-suite-with-fortran).
 There are additional configure-time options that can be used with the gfortran 
 tests. More details about those options and their purpose can be found in 
 [`Fortran/gfortran/README.md`](https://github.com/llvm/llvm-test-suite/tree/main/Fortran/gfortran/README.md).
-
- These tests are Free Software and are shared under the terms of the GNU General Public License (GPL). For more details, please see the accompanying [`LICENSE`](https://github.com/llvm/llvm-test-suite/tree/main/Fortran/gfortran/LICENSE.txt) file.