gdb/testsuite: remove F77_FOR_TARGET support

gdb/testsuite: remove F77_FOR_TARGET support

The last uses of the F77_FOR_TARGET via passing f77 to GDB's compile
procedure were removed in this commit

   commit 0ecee54cfd04a60e7ca61ae07c72b20e21390257
   Author: Tom Tromey <tromey@redhat.com>
   Date:   Wed Jun 29 17:50:47 2011 +0000

over 10 years ago.  The last .f files in the testsuite by now are all
being compiled by passing 'f90' to the GDB compile, thus only actually
using F90_FOR_TARGET (array-element.f, block-data.f, subarray.f).
Gfortran in this case is backwards compatible with most f77 code as
claimed on gcc.gnu.org/fortran.

The reason we'd like to get rid of this now is, that we'll be
implementing a Fortran compiler identification mechanism, similar to the
C/Cpp existing ones.  It would be using the Fortran preprocessor macro
defines to identify the Fortran compiler version at hand.  We found it
inconsequent to only implement this for f90 but, on the other hand, f77
seems deprecated.  So, with this commit we remove the remaining lines for
its support.