[llvm-dwarfdump] Remove unnecessary explicit -h behaviour

--help and -h are automatically supported by the command-line parser,
unless overridden by the tool. The behaviour of the PrintHelpMessage
being used for -h prior to this patch is subtly different to that
provided by --help automatically (it omits certain elements of help text
and options, such as --help-list), so overriding the default is not
desirable, without good reason. This patch removes the explicit
specification of -h and its behaviour, so that the default behaviour is
used.

Reviewed by: hintonda

Differential Revision: https://reviews.llvm.org/D63565

llvm-svn: 364029