Remove has_atexit

Also it wasn't even used in the first place, although it should have been.

Author: emmabastas

src/fpm_lock.f90

@@ -124,7 +124,6 @@ module fpm_lock
 public :: fpm_lock_acquire, fpm_lock_acquire_noblock, fpm_lock_release
 
 logical :: has_lock = .false.
-logical :: has_atexit_handler = .false.
 
 interface
     ! This function is defined in `fpm_lock.c`.