Remove Fortran compiler paths

spack.yaml

@@ -52,8 +52,8 @@ spack:
       paths:
         cc: /usr/bin/gcc
         cxx: /usr/bin/g++
-        f77: /usr/bin/gfortran
-        fc: /usr/bin/gfortran
+        f77: 
+        fc: 
       flags: {}
       operating_system: ubuntu18.04
       target: x86_64