libtoolize: use printf '%s\n' unconditionally.

It's been a year since the as_echo probes were removed in Autoconf,
so we can follow suit and remove our equivalent bs_echo probing
now.  Retain $ECHO in case users need to override default printf
calls in museum piece environments.
* gl/build-aux/funclib.sh (ECHO): Default to 'printf %s\n'.
(bs_echo): Remove.
Adjust all bs_echo callers to use $ECHO instead.
* bootstrap: Regenerate.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>