gdb/copyright.py: Remove testsuite/gdb.base/step-line.{c,inp} special handling

gdb/copyright.py: Remove testsuite/gdb.base/step-line.{c,inp} special handling

In the past, these files needed to be handled by hand, because
the testcase was sensitive to the length of the header, which was
potentially changing when new copyright years were added to
the copyright header. Now that we simply maintain and update
a range, the length of the copyright header should not change
as a consequence of the update performed by this script, so
special handling of those files is no longer necessary.

gdb/ChangeLog:

        * copyright.py (BY_HAND): Remove gdb/testsuite/gdb.base/step-line.inp
        and gdb/testsuite/gdb.base/step-line.c.