Remove outdated information from ERC manual

Remove outdated information from ERC manual

* doc/misc/erc.texi: Include emacsver.texi, and use EMACSVER rather than
ERC version.
(Introduction): ERC is distributed with Emacs.
(Obtaining ERC, Installation): Remove chapters, no longer relevant.
(Getting Started): Simplify.
(Getting Help and Reporting Bugs): Refer to general Emacs lists.
(History): Mention ERC maintained as part of Emacs now.

* doc/misc/Makefile.in ($(buildinfodir)/erc$(INFO_EXT), erc.dvi, erc.pdf):
Add dependency on emacsver.texi.