Fix post_process non-MPI builds never reading input file

In s_initialize_mpi_domain, the entire body including
s_assign_default_values_to_user_inputs(), s_read_input_file(), and
s_check_input_file() was guarded by #ifdef MFC_MPI. Non-MPI builds
therefore ran with all parameters at Fortran defaults (case_dir blank,
m/n/p = 0), causing a misleading "Time-step folder missing" error.

pre_process and simulation already call these unconditionally; the
individual MPI functions (s_mpi_bcast_user_inputs, etc.) handle the
#ifdef MFC_MPI guard internally as stubs when MPI is absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/post_process/m_start_up.fpp

@@ -1149,7 +1149,6 @@ contains
 
         num_dims = 1 + min(1, n) + min(1, p)
 
-#ifdef MFC_MPI
         ! Initialization of the MPI environment
         call s_mpi_initialize()
 
@@ -1167,14 +1166,12 @@ contains
 
         ! Broadcasting the user inputs to all of the processors and performing the
         ! parallel computational domain decomposition. Neither procedure has to be
-        ! carried out if the simulation is in fact not truly executed in parallel.
+        ! carried out if the post-process is in fact not truly executed in parallel.
         call s_mpi_bcast_user_inputs()
         call s_initialize_parallel_io()
         call s_mpi_decompose_computational_domain()
         call s_check_inputs_fft()
 
-#endif
-
     end subroutine s_initialize_mpi_domain
 
     !> @brief Destroy FFTW plans, free MPI communicators, and finalize all post-process sub-modules.