Update test/constants/test_constants.f90

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: MilanSkocic

test/constants/test_constants.f90

@@ -189,7 +189,6 @@ subroutine test_PLANCK_CONSTANT(error)
 end subroutine
 
 subroutine test_SPEED_OF_LIGHT(error)
-    implicit none
     type(error_type), allocatable, intent(out) :: error 
     real(dp) :: value, expected, diff, fac
     fac = 1.0d0