[gdb/testsuite] Remove superfluous marker in pr13961.S

[gdb/testsuite] Remove superfluous marker in pr13961.S

When looking at readelf output for test-case gdb.dwarf2/pr13961.exp, we have:
...
$ readelf -wi ./outputs/gdb.dwarf2/pr13961/pr13961
   ...
 <0><14c>: Abbrev Number: 0
readelf: Warning: Bogus end-of-siblings marker detected at offset 14c in \
  .debug_info section
...

Fix this by removing the superfluous end-of-siblings marker.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-20  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/pr13961.S: Remove superfluous end-of-siblings marker.