core: Fixup assert-max-image-size/assert-max-file-size usage

- Remove the incorrectly used (and unused) 3rd arg to assert-max-image-size
- Remove test for fstype in assert-max-file-size() as it was always
  set to a value leading to 0.

Change-Id: I0a1182fcc85c6c3801d45731e691b9bb38657606
Signed-off-by: JP Abgrall <jpa@google.com>