Remove gdb.base/branches.c

This patch removes gdb.base/branches.c which was added by the following
commit, but it is not used at all.

commit ea8122af
Author: John Metzler <jmetzler@cygnus>
Date:   Thu Apr 16 17:56:11 1998 +0000

    Thu Apr 16 10:52:34 1998  John Metzler  <jmetzler@cygnus.com>

        * gdb.base/branches.c: Code with lots of loops and
        subroutines. Used to test gdbs ability to single step through PC
        changes, especially to test mips-tdep.c:mips_next_pc

gdb/testsuite:

2016-02-25  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/branches.c: Remove.