bootstrap: remove some now-unneeded code

bootstrap: remove some now-unneeded code

This script arose back when gnulib-tool was young.
Since then, it has seen improvements that render much of this
script unnecessary.  In particular, it can now make symlinks
to the files it uses.  Also, I no longer see as much value in
marking files as read-only via comments.
If you relied on the symlink-creation feature of the preceding
version of this script, you can get most of that functionality
by adding the --symlink option to the definition of
gnulib_tool_option_extras in your bootstrap.conf file.
* build-aux/bootstrap (AUTOPOINT, AUTORECONF): Factor out definitions.
Run autopoint and libtoolize *before* gnulib-tool.
After it, run an abbreviated autoreconf, rather than a loop around
all tools.
(slirp, bt_mark_as_generated): Remove functions.