Revert "Juan 13/05/2019:Makefile , force internal SHELL=/bin/bash , for ubuntu/dash problem"

This reverts commit ea5e3b34.

src/Makefile

@@ -2,7 +2,6 @@
 #MNH_LIC This is part of the Meso-NH software governed by the CeCILL-C licence
 #MNH_LIC version 1. See LICENSE, CeCILL-C_V1-en.txt and CeCILL-C_V1-fr.txt
 #MNH_LIC for details. version 1.
-SHELL='/bin/bash'
 ##########################################################
 #                                                        #
 #  MACRO : dependance_flist                              #