WarpX class: simplify return type of get_spectral_solver_fp using `au…

…to&` (#5656)

This PR simplifies the return type of a method of the WarpX class by
replacing:
```
#   ifdef WARPX_DIM_RZ
        SpectralSolverRZ&
#   else
        SpectralSolver&
#   endif
```
with
```
auto&
```

Source/WarpX.H

@@ -1005,13 +1005,7 @@ public:
     void PSATDSubtractCurrentPartialSumsAvg ();
 
 #ifdef WARPX_USE_FFT
-
-#   ifdef WARPX_DIM_RZ
-        SpectralSolverRZ&
-#   else
-        SpectralSolver&
-#   endif
-            get_spectral_solver_fp (int lev) {return *spectral_solver_fp[lev];}
+    auto&  get_spectral_solver_fp (int lev) {return *spectral_solver_fp[lev];}
 #endif
 
     FiniteDifferenceSolver * get_pointer_fdtd_solver_fp (int lev) { return m_fdtd_solver_fp[lev].get(); }