removing libreadline from base system

[Max Khon]

Hello!

It is possible to build and link our in-tree gdb & friends with libedit
after r228114.

The remaining question is what to do with libreadline:

1) just build & link gdb with libedit

OR

2) re-import libreadline from gdb sources and build INTERNALLIB version of
it that is never installed and is linked only to gdb

I am inclined to go for 1) but libedit may have (and has) incompatibilities
with libreadline.

Max