Remove unused variables in procfs.c etc.

The recent patch to enable -Wunused-variable uncovered a couple of
instances in Solaris-specific files:

/vol/src/gnu/gdb/hg/master/dist/gdb/proc-api.c: In function ‘void _initialize_proc_api()’:
/vol/src/gnu/gdb/hg/master/dist/gdb/proc-api.c:422:28: error: unused variable ‘c’ [-Werror=unused-variable]
   struct cmd_list_element *c;
                            ^

/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c: In function ‘void procfs_init_inferior(target_ops*, int)’:
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c:2895:12: error: unused variable ‘signals’ [-Werror=unused-variable]
   sigset_t signals;
            ^~~~~~~
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c: In member function ‘virtual char* procfs_target::make_corefile_notes(bfd*, int*)’:
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c:3845:19: error: unused variable ‘old_chain’ [-Werror=unused-variable]
   struct cleanup *old_chain;
                   ^~~~~~~~~
/vol/src/gnu/gdb/hg/master/dist/gdb/procfs.c:3847:18: error: unused variable ‘fpregs’ [-Werror=unused-variable]
   gdb_fpregset_t fpregs;
                  ^~~~~~

Fixed as follows.  Tested on amd64-pc-solaris2.11, installed as obvious.

	* proc-api.c (_initialize_proc_api): Remove c, unused.
	* procfs.c (procfs_init_inferior): Remove signals, unused.
	(procfs_target::make_corefile_notes): Remove old_chain, fpregs,
	unused.