cleanup

[matsl]

What

• Cut dependency cycle
• Remove duplicate defvar
• Remove redundant requires

Why

Running latest Emacs (tip version) with no byte compilation gives the
following warning message:

 stop  Warning (emacs): loaddefs-gen: load error
	(error Eager macro-expansion failure: (error "Eager macro-expansion skipped due to cycle:
  … => (load \"hbut.el\") => (macroexpand-all …) => (macroexpand (eval-and-compile …)) => (load \"hbdata.el\") => (load \"hgnus.el\") => (load \"hvar.el\") => (load \"hsettings.el\") => (load \"hproperty.el\") => (load \"hbut.el\")"))

This dependency is cut in the PR by using declare-function.

The duplicated defvar was found when looking at this issue.

[rswgnu]

• Fix conflicts.