Remove relational operators from common/offset-type.h

Remove relational operators from common/offset-type.h

This patch is a follow-up of:

  https://sourceware.org/ml/gdb-patches/2018-10/msg00601.html

It removes the declaration of the relational operators for
common/offset-type.h.  As it turns out, these overloads are not being
used when a new offset type is declared, because, according to Pedro
Alves:

  I think the functions aren't called because they are templates, and
  thus the built-in (non-template) versions take precedence.  If you
  make them non-templates, then they should be called.  But, the
  built-ins are fine, so yeah, we can just remove the custom
  definitions.

The patch also adjusts the comments on the code.

No regressions introduced.

gdb/ChangeLog:
2018-10-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* common/offset-type.h (DEFINE_OFFSET_REL_OP): Delete.
	Adjust comments.