gdb: remove the silent parameter from exit_inferior_1 and cleanup

After the previous commit, exit_inferior_1 no longer makes use of the
silent parameter.  This commit removes this parameter and cleans up
the callers.

After doing this exit_inferior_1, exit_inferior, and
exit_inferior_silent are all equivalent, so rename exit_inferior_1 to
exit_inferior and delete exit_inferior_silent, update all the callers.

Also I spotted the declaration exit_inferior_num_silent in inferior.h,
but this function is not defined anywhere, so I deleted the
declaration.

There should be no user visible changes after this commit.